#include <bits/stdc++.h>
using namespace std;
vector<string> ans;
void tr(string a, string b){
int cn[3];
cn[0] = cn[1] = cn[2] = 0;
while(cn[0] < (int)a.size() && cn[1] < (int)b.size()){
if(a[cn[0]] == b[cn[1]]){cn[0]++, cn[1]++;}
else{cn[2]++; cn[1]++;}
}
if(cn[2] > 1)return;
if(ans.empty()){ans.push_back(a);return;}
if(ans[0] != a){ans.push_back(a);return;}
}
int main(){
int n; cin >> n;
string s; cin >> s;
if(!(n&1)){cout << "NOT POSSIBLE\n";return 0;}
n/=2;
tr(s.substr(0,n),s.substr(n,n+1));
tr(s.substr(n+1,n), s.substr(0,n+1));
if(ans.empty()){cout << "NOT POSSIBLE\n";return 0;}
if(ans.size() == 1){cout << ans[0] << "\n"; return 0;}
cout << "NOT UNIQUE\n";
}
