#include <bits/stdc++.h>
using namespace std;
const int MN = 2e6+6;
int h[MN], pw[MN]={1}, N, L, i, cnt;
string s, ans; set<int> lol;
int hsh(int l,int r){return h[r]-h[l-1]*pw[r-l+1];}
int main(){
cin >> N >> s;
for(i=1;i<=N;i++){
h[i]=h[i-1]*131+s[i-1]-'A';
pw[i]=pw[i-1]*131;
}
if(N%2){
L = N/2;
for(i=1;i<=N;i++){
if(i<L+1){
int l1 = i-1;
int h1 = hsh(1,l1)*pw[L-l1]+hsh(i+1,i+L-l1);
int h2 = hsh(N-L+1,N);
if(h1==h2){
cnt++; lol.insert(hsh(N-L+1,N));
if(cnt == 1){
for(int j=N-L+1;j<=N;j++)
ans.push_back(s[j-1]);
}
}
}
else if(i>L+1){
int h1 = hsh(1,L);
int h2 = hsh(L+1,i-1)*pw[N-i]+hsh(i+1,N);
if(h1==h2){
cnt++; lol.insert(hsh(1,L));
if(cnt == 1){
for(int j=1;j<=L;j++)
ans.push_back(s[j-1]);
}
}
}
else{
if(hsh(1,i-1)==hsh(i+1,N)){
cnt++; lol.insert(hsh(1,L));
if(cnt == 1){
for(int j=1;j<=L;j++)
ans.push_back(s[j-1]);
}
}
}
}
if(cnt==0) printf("NOT POSSIBLE\n");
else if(lol.size()>1) printf("NOT UNIQUE\n");
else printf("%s\n",ans.c_str());
}
else printf("NOT POSSIBLE\n");
return 0;
}
