#include <bits/stdc++.h>
#define int long long
#define pb push_back
#define F first
#define S second
#define all(a) a.begin(),a.end()
#define pii pair <int,int>
#define int long long
using namespace std ;
mt19937 rng(time(0)) ;
const int maxn = 2e6 + 10 , pr[] = {71 , 53};
int pre[2][maxn] , pw[2][maxn] , mod[2] , ipw[2][maxn] ;
int po(int a, int b , int m ){
if(b==0)return 1;
int v= po(a , b/2 , m) ;
v = v*v % m ;
if(b&1) v = v*a % m ;
return v;
}
int hs(int i , int l , int r){
return (pre[i][r] - pre[i][l-1] + mod[i]) * ipw[i][l-1] % mod[i] ;
}
signed main(){
ios_base::sync_with_stdio(false); cin.tie(NULL);
int n ;
string s;
cin >> n >> s ;
if(n % 2==0){
cout << "NOT POSSIBLE\n";
return 0 ;
}
vector <int> prime ;
for(int i = 1e9 ; i <= 1e9 + 200 ; i++){
bool ok = 1;
for(int j = 2; j*j <= i ; j++){
if(i%j==0)ok = 0 ;
}
if(ok)prime.pb(i) ;
}
mod[0] = prime[rng() % (int)prime.size()] ;
mod[1] = prime[rng() % (int)prime.size()] ;
for(int i =0; i < 2; i++){
pw[i][0] = 1;
for(int j = 1; j < maxn ; j++){
pw[i][j] = pw[i][j-1] * pr[i] % mod[i] ;
}
ipw[i][maxn-1] = po(pw[i][maxn-1] , mod[i]-2 , mod[i]) ;
for(int j = maxn-2 ; j >= 0; j --){
ipw[i][j] = ipw[i][j+1] * pr[i] % mod[i] ;
}
}
s = " " + s;
for(int i = 0 ; i < 2; i ++){
for(int j =1 ; j <= n ; j++){
pre[i][j] = (pre[i][j-1] + ((int)(s[j]-'A') + 1) * pw[i][j] % mod[i]) % mod[i] ;
}
}
vector <pair <int,pii> > vec;
for(int i =1 ; i <= n ; i++){
bool ok =1;
int a[3] ;
a[0] = a[1] = 0 ;
for(int j =0 ; j < 2 ; j++){
if(i == n/2+1 && hs(j , 1 , i-1) != hs(j , i+1 , n)){
ok = 0;
}
if(i <= n/2 && hs(j , n/2+2 , n) != (hs(j , 1 , i-1) + hs(j , i+1 , n/2+1) * pw[j][i-1] % mod[j])%mod[j]){
ok =0 ;
}
if(i >= n/2+2 && hs(j , 1 , n/2) != (hs(j , n/2+1 , i-1) + hs(j , i+1 , n) * pw[j][i-1] % mod[j])%mod[j]){
ok = 0 ;
}
if(i <= n/2+1){
a[j] = hs(j , n/2+2 , n) ;
}else{
a[j]= hs(j , 1 , n/2) ;
}
}
if(ok == 1){
vec.pb({i , {a[0] , a[1]}}) ;
}
}
bool ok =0 ;
for(int i = 1; i <vec.size() ; i++){
if(vec[i].S != vec[i-1].S) ok = 1;
}
if(vec.size() ==0){
cout << "NOT POSSIBLE\n";
return 0 ;
}
if(ok){
cout << "NOT UNIQUE\n";
return 0 ;
}
string a ;
for(int i = 1; i <= n ; i++){
if(i == vec[0].F)continue ;
a += s[i] ;
}
for(int i =0 ; i < n/2 ;i++){
cout << a[i] ;
}
}
