#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 = 2e6 + 7 , mod = 1e9 + 7 ;
const ll inf = 2e18 ;
// How interesting!
int n ;
string s;
int answer = 0 ;
int pref1[N] , pref2[N] ;
inline bool good1(int l , int r){if(l>r)return 1; return pref1[r] - (l ? pref1[l-1] : 0) == r - l + 1 ; }
inline bool good2(int l , int r){if(l>r)return 1; return pref2[r] - (l ? pref2[l-1] : 0) == r - l + 1; }
int main(){
ios_base::sync_with_stdio(0) ;
cin.tie(0) ;
//freopen("in.in" , "r" , stdin) ;
cin >> n;
cin >> s;
int ans = -1 ;
set<string> vec ;
for(int i = 0 ;i <= n / 2 ; ++ i){
pref1[i] = (s[i] == s[i+n/2]) ;
pref2[i] = (s[i] == s[i+n/2+1]) ;
if(i){
pref1[i]+=pref1[i-1] ;
pref2[i]+=pref2[i-1] ;
}
}
if(n&1){
if(s == string(n , s[0])){
return cout<< s.substr(0 , n / 2) , 0 ;
}
for(int i = 0 ;i < n ; ++ i){
bool ok = 1 ;
if(i>=n/2){
int j = i - n / 2;
ok&=good1(0 , j - 1) ;
ok&=good2(j,n/2-1) ;
}else{
ok&=good1(i + 1, n / 2) ;
ok&=good2(0 , i - 1) ;
}
if(ok){
string t = s ;
t.erase(i , 1) ;
vec.insert(t.substr(0 , n /2)) ;
}
}
if(vec.size() == 1){
cout<< *vec.begin() ;
}else if(vec.size() > 1)
cout<<"NOT UNIQUE" ;
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.
*/
