#include <bits/stdc++.h>
using namespace std;
const int MAXN = 2e6+5;
char arr[MAXN];
int main()
{
ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
int N; cin >> N;
string U; cin >> U;
if(N % 2 == 0){
cout << "NOT POSSIBLE\n";
return 0;
}
for(int i = 0;i<N;++i){
arr[i+1] = U[i];
}
int cnt = 0, upp = (N+1)/2;
for(int i = 2;i<=upp;++i){
if(arr[i] == arr[i+upp-1]){
++cnt;
}
}
int pos1 = -1;
if(cnt == upp-1){
pos1 = 1;
}
for(int i = 2;i<=upp-1;++i){
if(arr[i-1] == arr[upp+i-1]){
++cnt;
}
if(arr[i] == arr[upp+i-1]){
--cnt;
}
if(cnt == upp-1){
pos1 = i;
break;
}
}
cnt = 0; int pos2 = -1;
for(int i = 1;i<=upp-1;++i){
if(arr[i] == arr[upp+i]){
++cnt;
}
}
if(cnt == upp-1){
pos2 = upp;
}
for(int i = upp+1;i<=N;++i){
if(arr[i-upp] == arr[i-1]){
++cnt;
}
if(arr[i-upp] == arr[i]){
--cnt;
}
if(cnt == upp-1){
pos2 = i;
break;
}
}
if(pos1 != -1 && pos2 != -1){
bool same = true;
for(int i = 1;i<=upp-1;++i){
if(arr[i] != arr[upp+i]){
same = false;
}
}
if(same){
for(int i = 1;i<=upp-1;++i){
cout << arr[i];
}
cout << "\n";
}
else{
cout << "NOT UNIQUE\n";
}
}
else if(pos1 != -1){
for(int i = upp+1;i<=N;++i){
cout << arr[i];
}
cout << "\n";
}
else if(pos2 != -1){
for(int i = 1;i<=upp-1;++i){
cout << arr[i];
}
cout << "\n";
}
else{
cout << "NOT POSSIBLE\n";
}
return 0;
}
