#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cstring>
#include <cmath>
#include <queue>
#include <map>
#define ll  long long
#define out(a) printf("%d ",a)
#define writeln printf("")
const int N=2e6+50;
const int MOD=1e9+7;
const int base=233;
using namespace std;
char s[N];
int n,len,id;
int num,ans,cnt=0;
bool f,flag;
ll Hash[N],Pow[N];
int read()
{
    int s=0,t=1; char c;
    while (c<'0'||c>'9'){if (c=='-') t=-1; c=getchar();}
    while (c>='0'&&c<='9'){s=s*10+c-'0'; c=getchar();}
    return s*t;
}
ll readl()
{
    ll s=0,t=1; char c;
    while (c<'0'||c>'9'){if (c=='-') t=-1; c=getchar();}
    while (c>='0'&&c<='9'){s=s*10+c-'0'; c=getchar();}
    return s*t;
}
ll get(int l,int r)
{
    if (l<=r) return (Hash[r]-(ll)Hash[l-1]*Pow[r-l+1]%MOD+MOD)%MOD;
    else return 0;
}
ll find(int x)
{
    ll y1=0,y2=0;
    if (x<=(n>>1)) y1=(get(0,x-1)*Pow[(n>>1)-x]%MOD+get(x+1,n>>1))%MOD,y2=get((n>>1)+1,n-1);
    else y1=get(0,(n>>1)-1),y2=(get((n>>1),x-1)*Pow[n-x-1]%MOD+get(x+1,n-1))%MOD;
    //out(y1); out(y2); writeln;
    if (y1==y2) return y1;
    else return 0;
}
int main()
{
    n=read();
    if (!(n&1)) {
      puts("NOT POSSIBLE");
      return 0;
    }
    else {
      scanf("%s",s);
      len=strlen(s); f=flag=false;
      Pow[0]=1;
      for (int i=1;i<=len;i++)
        Pow[i]=Pow[i-1]*base%MOD;
      for (int i=0;i<len;i++)
        Hash[i]=(Hash[i-1]*base+s[i])%MOD;
      for (int i=0;i<len;i++){
        num=find(i);
        if (num) {
            //out(233); writeln;
          if (!f) ans=num,id=i;
          else if (ans!=num) {
              flag=true; break;
          }
          f=true;
        }
      }
      if (f) {
        if (flag) puts("NOT UNIQUE");
        else {
          for (int i=0;i<len;i++){
            if (i!=id) putchar(s[i]),cnt++;
            if (cnt>=(n>>1)) break;
          }
        }
      }
      else puts("NOT POSSIBLE");
    }
      return 0;
}