#include<queue>
#include<math.h>
#include<stdio.h>
#include<iostream>
#include<vector>
#include<iomanip>
#include<string.h>
#include<algorithm>
#include<cmath>
#include<cstdio>
#include<utility>
#include<cstring>
#include<stack>
#include<fstream>
#include<string>
using namespace std;
typedef long long ll;
const int N = 1e5 + 10;
const int maxn = 5e2 + 10;
const int INF = 0x3f3f3f3f;
struct rec
{
    int x , y , lie;
};
int n , m;
char Map[maxn][maxn];
int step[maxn][maxn][3];
int dir[4][2] = { 0 , -1 , 0 , 1 , -1 , 0 , 1 , 0 };
int next_x[3][4] = { { 0 , 0 , -2 , 1 } , { 0 , 0 , -1 , 1 } , { 0 , 0 , -1 , 2 } };
int next_y[3][4] = { { -2 , 1 , 0 , 0 } , { -1 , 2 , 0 , 0 } , { -1 , 1 , 0 , 0 } };
int next_lie[3][4] = { { 1, 1, 2 , 2 } , { 0 , 0 , 1 , 1 } , { 2 , 2 , 0 , 0 } }; 
struct rec st , ed;
bool valid( int x , int y )
{
    if ( x <= 0 || x > n || y <= 0 || y > m )
	{   
        return false;
    }
    return true;
}
void parse_st_ed()
{
    for ( int i = 1 ; i <= n ; i++ )
	{
        for ( int j = 1 ; j <= m ; j++ )
		{
            if ( Map[i][j] == 'O' )
			{
                ed.x = i;
                ed.y = j;
                ed.lie = 0;
                Map[i][j] = '.';
            }
            if ( Map[i][j] == 'X' )
			{
                for ( int k = 0 ; k < 4 ; k++ )
				{
                    int dx = dir[k][0] + i;
                    int dy = dir[k][1] + j;
                    if ( valid( dx , dy ) && Map[dx][dy] == 'X' )
					{
                        st.x = min( dx , i );
                        st.y = min( dy , j );
                        st.lie = k < 2 ? 1 : 2;
                        Map[i][j] = Map[dx][dy] = '.';
                        break;
                    }
                }
                if ( Map[i][j] == 'X' )
				{
                    st.x = i;
                    st.y = j;
                    st.lie = 0;
                    Map[i][j] = '.';
                }
            }
        }
    }
}
bool check( rec next )
{
    if ( !valid( next.x , next.y ) )
	{
        return false;
    }
    if ( Map[next.x][next.y] == '#' )
	{
        return false;
    }
    if ( next.lie == 0 && Map[next.x][next.y] != '.' )
	{
        return false;
    }
    if ( next.lie == 1 && Map[next.x][ next.y + 1 ] == '#' )
	{
        return false;
    }
    if ( next.lie == 2 && Map[ next.x + 1 ][next.y] == '#' )
	{
        return false;
    }
    return true;
}
int bfs()
{
    for ( int i = 1 ; i <= n ; i++ )
	{
        for ( int j = 1 ; j <= m ; j++ )
		{
            for ( int k = 0 ; k < 3 ; k++ )
			{
                step[i][j][k] = -1;
            }
        }
    }
    queue<rec> q;
    step[st.x][st.y][st.lie] = 0;
    q.push(st);
    while ( !q.empty() )
	{
        rec now = q.front();
        q.pop();
        for ( int i = 0 ; i < 4 ; i++ )
		{
            rec next;
            next.x = now.x + next_x[now.lie][i];
            next.y = now.y + next_y[now.lie][i];
            next.lie = next_lie[now.lie][i];
            if ( check(next) && step[next.x][next.y][next.lie] == -1 )
			{
                step[next.x][next.y][next.lie] = step[now.x][now.y][now.lie] + 1;
                q.push(next);
                if ( next.x == ed.x && next.y == ed.y && next.lie == ed.lie )
				{
                    return step[next.x][next.y][next.lie];
                }
            }
        }
    }
    return -1;
}
int main()
{
    while ( ~scanf( "%d %d" , &n , &m ) )
	{
        if ( n == 0 && m == 0 )
		{
            break;
        }
        for ( int i = 1 ; i <= n ; i++ )
		{
            for ( int j = 1 ; j <= m ; j++ )
			{
                cin >> Map[i][j];
            }
        }
        parse_st_ed();
        int ans = bfs();
        if ( ans != -1 )
		{
            printf( "%d\n" , ans );
        }
		else
		{
            printf( "Impossible\n" );
        }
    }
    return 0;
}

C++ :

#include <iostream>
#include <cstdio>
#include <queue>

using namespace std;

const int maxn = 5e2+7;

struct rec {
    int x, y, lie;
};

int n, m;
char Map[maxn][maxn];
int step[maxn][maxn][3];       //存储步数
int dir[4][2] = {0, -1, 0, 1, -1, 0, 1, 0};     //普通方向数组
//设0是立着，1是横着，2是竖着，这三种状态
int next_x[3][4] = {{0, 0, -2, 1}, {0, 0, -1, 1}, {0, 0, -1, 2}};
int next_y[3][4] = {{-2, 1, 0, 0}, {-1, 2, 0, 0}, {-1, 1, 0, 0}};
int next_lie[3][4] = {{1, 1, 2, 2}, {0, 0, 1, 1}, {2, 2, 0, 0}}; 
struct rec st, ed;  //分别是起点和终点位置

bool valid(int x, int y) {  //判断是否越界
    if(x <= 0 || x > n || y <= 0 || y > m) {   
        return false;
    }
    return true;
}

void parse_st_ed() {    //找起点和终点
    for(int i = 1; i <= n; i++) {
        for(int j = 1; j <= m; j++) {
            if(Map[i][j] == 'O') {
                ed.x = i;
                ed.y = j;
                ed.lie = 0;
                Map[i][j] = '.';
            }
            if(Map[i][j] == 'X') {
                for(int k = 0; k < 4; k++) {
                    int dx = dir[k][0] + i;
                    int dy = dir[k][1] + j;
                    if(valid(dx, dy) && Map[dx][dy] == 'X') {
                        st.x = min(dx, i);
                        st.y = min(dy, j);
                        st.lie = k < 2 ? 1 : 2; //在dir数组里，0和1是左右，2和3是上下
                        Map[i][j] = Map[dx][dy] = '.';
                        break;
                    }
                }
                if(Map[i][j] == 'X') {
                    st.x = i;
                    st.y = j;
                    st.lie = 0;
                    Map[i][j] = '.';
                }
            }
        }
    }
}

bool check(rec next) {
    if(!valid(next.x, next.y)) {
        return false;
    }
    if(Map[next.x][next.y] == '#') {
        return false;
    }
    //判断当前位置是否可行
    if(next.lie == 0 && Map[next.x][next.y] != '.') {
        return false;
    }
    if(next.lie == 1 && Map[next.x][next.y + 1] == '#') {
        return false;
    }
    if(next.lie == 2 && Map[next.x + 1][next.y] == '#') {
        return false;
    }
    return true;
}

int bfs() {
    for(int i = 1; i <= n; i++) {
        for(int j = 1; j <= m; j++) {
            for(int k = 0; k < 3; k++) {
                step[i][j][k] = -1;
            }
        }
    }
    queue<rec> q;
    step[st.x][st.y][st.lie] = 0;
    q.push(st);
    while(!q.empty()) {
        rec now = q.front();
        q.pop();
        for(int i = 0; i < 4; i++) {
            rec next;
            next.x = now.x + next_x[now.lie][i];
            next.y = now.y + next_y[now.lie][i];
            next.lie = next_lie[now.lie][i];
            if(check(next) && step[next.x][next.y][next.lie] == -1) {
                step[next.x][next.y][next.lie] = step[now.x][now.y][now.lie] + 1;
                q.push(next);
                if(next.x == ed.x && next.y == ed.y && next.lie == ed.lie) {
                    return step[next.x][next.y][next.lie];
                }
            }
        }
    }
    return -1;  //无解
}

int main() {
    while(~scanf("%d %d", &n, &m)) {
        if(n == 0 && m == 0) {
            break;
        }
        for(int i = 1; i <= n; i++) {
            for(int j = 1; j <= m; j++) {
                cin >> Map[i][j];
            }
        }
        parse_st_ed();
        int ans = bfs();
        if(ans != -1) {
            printf("%d\n", ans);
        } else {
            printf("Impossible\n");
        }
    }
    
    return 0;
}