#include <queue>
#include <math.h>
#include <stack>
#include <stdio.h>
#include <iostream>
#include <vector>
#include <iomanip>
#include <string.h>
#include <algorithm>
using namespace std;
#define int long long
#define void inline void
queue<int> Q;
stack<int> S;
vector<int> V;
#ifndef SOMETHING_H
#define SOMETHING_H
#endif
#define int long long
inline int read(){
	char c = getchar();
	int s = 0;
	while(c < '0' || c > '9')
		c = getchar();
	while(c >= '0' && c <= '9'){
		s = s * 10 + c - '0';
		c = getchar();
	}
	return s;
}
int n, m, ans[52][52][52][52], a[52][52];
inline int f(int x1, int y1, int x2, int y2){
	if(x1 == n && x2 == n && y1 == m && y2 == m)
		return 0;
	if(ans[x1][y1][x2][y2] != -1)
		return ans[x1][y1][x2][y2];
	int kkk = 0;
	if(x1 < n && x2 < n)
		if(a[x1 + 1][y1] != -1 && a[x2 + 1][y2] != -1)
			kkk = max(kkk, f(x1 + 1, y1, x2 + 1, y2) + a[x1 + 1][y1] + a[x2 + 1][y2] - a[x1 + 1][y1] * (x1 == x2 && y1 == y2));
	if(y1 < m && y2 < m)
		if(a[x1][y1 + 1] != -1 && a[x2][y2 + 1] != -1)
			kkk = max(kkk, f(x1, y1 + 1, x2, y2 + 1) + a[x1][y1 + 1] + a[x2][y2 + 1] - a[x1][y1 + 1] * (x1 == x2 && y1 == y2));
	if(x1 < n && y2 < m)
		if(a[x1 + 1][y1] != -1 && a[x2][y2 + 1] != -1)
			kkk = max(kkk, f(x1 + 1, y1, x2, y2 + 1) + a[x1 + 1][y1] + a[x2][y2 + 1] - a[x1 + 1][y1] * (x1 + 1 == x2 && y1 == y2 + 1));
	if(y1 < m && x2 < n)
		if(a[x1][y1 + 1] != -1 && a[x2 + 1][y2] != -1)
			kkk = max(kkk, f(x1, y1 + 1, x2 + 1, y2) + a[x1][y1 + 1] + a[x2 + 1][y2] - a[x1][y1 + 1] * (x1 == x2 + 1 && y1 + 1 == y2));
	ans[x1][y1][x2][y2] = kkk;
	return kkk;
}
void Main(){
	n = read();
	m = read();
	memset(ans, -1, sizeof(ans));
	for(int i = 1; i <= n; i++)
		for(int o = 1; o <= m; o++)
			a[i][o] = read();
	cout << f(1, 1, 1, 1);
	return;
}
signed main(int argc, char **argv){
	Main();
	return 0;
}

#include <iostream>

using namespace std;
const int N = 60;
int a[N][N];
int dp[N][N][N][N];
int main(){
	int n, m;
	cin >> n >> m;
	for(int i = 1; i <= n; i++){
		for(int j = 1; j <= m; j++){
			cin >> a[i][j];
		}
	}
	for(int i = 1; i <= n; i++){
		for(int j = 1; j <= m; j++){
			for(int x = 1; x <= n; x ++){
				for(int y = 1; y <= m; y++){
					int *t;
					t = &dp[i][j][x][y];
					*t = max(*t, dp[i-1][j][x][y-1]);
					*t = max(*t, dp[i-1][j][x-1][y]);
					*t = max(*t, dp[i][j-1][x-1][y]);
					*t = max(*t, dp[i][j-1][x][y-1]);
					*t += a[i][j] + a[x][y];
					if(i == x && j == y)
						*t -= a[x][y];
				}
			}
		}
	
	}
	cout << dp[n][m][n][m];
} 

#include<bits/stdc++.h>
using namespace std;
const int N=55;
int m,n,a[N][N],dp[N][N][N];
int main(){
    cin>>m>>n;
    for(int i=1;i<=m;i++){
        for(int j=1;j<=n;j++){
            cin>>a[i][j];
        }
    }
    for(int i=1;i<=m;i++)
        for(int j=1;j<=n;j++)
            for(int k=1;k<=m;k++){
                int l=i+j-k;
                if(l>=1&&l<=n) {
                    int temp=max(max(dp[i-1][j][k-1],dp[i-1][j][k]),max(dp[i][j-1][k-1],dp[i][j-1][k]));
                    dp[i][j][k]=temp+a[i][j];
                    if(i!=k) dp[i][j][k]+=a[k][l];
                }
            }
    cout<<dp[m][n][m]<<endl;
    return 0;
}

C++ :

#include<bits/stdc++.h>
using namespace std;
int a[51][51],n,m;
unsigned int s[51][51][51][51];
int main(int argc,char*argv[]){
    memset(a,0,sizeof(a));
    memset(s,0,sizeof(s));
    scanf("%d%d",&m,&n);
    for(int i=1;i<=m;i++){
    	for(int j=1;j<=n;j++){
    		 scanf("%d",&a[i][j]); 
		}   
		int a1,a2;
    	for(int j=1;j<=n;j++){
    		for(int k=1;k<=m;k++){
    			for(int l=1;l<=n;l++){
            		a1=max(s[i][j-1][k][l-1],s[i-1][j][k][l-1]);
            		a2=max(s[i-1][j][k-1][l],s[i][j-1][k-1][l]);
            		s[i][j][k][l]=max(a1,a2)+a[i][j]+a[k][l];
            		if(i==k&&j==l)s[i][j][k][l]-=a[i][j];
        		}
			}
		}
	}
    printf("%d",s[m][n][m][n]);
    return 0;
}