#include<bits/stdc++.h>
#pragma GCC optimize("-Ofast")
//#pragma GCC optimize("trapv")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,sse4.2,popcnt,abm,mmx,avx2,tune=native")
#pragma GCC optimize("-ffast-math")
#pragma GCC optimize("-funroll-loops")
#define I inline void
#define S struct
#define vi vector<int>
#define vii vector<pair<int,int>>
#define pii pair<int,int>
#define pll pair<ll,ll>
using namespace std ;
using ll = long long ;
using ld = long double ;
const int N = 2e5 + 7 , mod = 1e9 + 7 ;
const ll inf = 2e18 ;
// How interesting!
int n ;
string s;
int answer = 0 ;
int main(){
ios_base::sync_with_stdio(0) ;
cin.tie(0) ;
//freopen("in.in" , "r" , stdin) ;
cin >> n;
cin >> s;
int ans = -1 ;
if(n&1){
for(int i = 0 ;i < n ; ++ i){
bool ok = 1 ;
if(i>=n/2){
int j = i - n / 2;
for(int k = 0 ; k < j ; ++ k){
ok&=(s[k] == s[k+n/2]);
}
for(int k = j ; k < n / 2 ; ++ k){
ok&=(s[k] == s[k+n/2+1]) ;
}
}else{
for(int j = 0 ;j <= n / 2 ; ++ j){
if(i == j)continue ;
ok&=(s[j] == s[j + n / 2 + (j<i)]) ;
}
}
if(ok){
if(~ans){
cout<<"NOT UNIQUE" ;
return 0 ;
}
ans = i ;
}
}
if(~ans){
s.erase(ans , 1) ;
cout<< s.substr(0 , n / 2) ;
}else cout<<"NOT POSSIBLE" ;
}
else cout<<"NOT POSSIBLE" ;
return 0 ;
}
/*
- bounds sir (segtree = 4N, eulerTour = 2N, ...)
- a variable defined twice?
- will overflow?
- is it a good complexity?
- don't mess up indices (0-indexed vs 1-indexed)
- reset everything between testcases.
*/
