#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#include <bits/stdc++.h>
using namespace std;
#define ll __int128
#define maxn 300001
#define INF 1000000001
#define pa pair<ll,ll>
#define vi(t) vector<t>
//long long T[1000002];
long SS[2000002];
long Left[2000002];
long Right[2000002];
int main(){
long N;
cin>>N;
for(long i = 1;i<=N;i++){
char temp;
cin>>temp;
SS[i] = (long)temp;
}
if (N%2==0) {
cout<<"NOT POSSIBLE";
return 0;
}
long j = N/2 + 1;
long used = 0;
for (long i = 1;i<=N/2;i++){
Left[i] = SS[i];
while(SS[i]!=SS[j]){
j++;
used++;
if (used==2){
Left[0] = 1;
break;
}
}
if (used==2){
Left[0] = 1;
break;
}
j++;
}
//cout<<used;
j = 1;
used = 0;
for (long i = 1;i<=N/2;i++){
Right[i] = SS[i+N/2+1];
while(SS[i+N/2+1]!=SS[j]){
j++;
used++;
if (used==2){
Right[0] = 2;
break;
}
}
if (used==2){
Right[0] = 2;
break;
}
j++;
}
if (Right[0]==2 && Left[0]==1){
cout<<"NOT POSSIBLE";
return 0 ;
}
if (Right[0]==2){
for (long i = 1;i<=N/2;i++) cout<<(char)Left[i];
return 0;
}
if (Left[0]==1){
for (long i = 1;i<=N/2;i++) cout<<(char)Right[i];
return 0;
}
for (long i = 1;i<=N/2;i++) if (Left[i]!=Right[i]){
cout<<"NOT UNIQUE";
return 0;
}
for (long i = 1;i<=N/2;i++) cout<<(char)Right[i];
}
