#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<ll,ll> ii;
const ll MOD1 = 1e9 + 9;
const ll prime1 = 999983;
const ll invprime1 = 943912055;
const int MAXN = 200011;
ll N,hash1[MAXN],pot1[MAXN],invpot1[MAXN];
vector<ll> possiveis;
set<ll> valores;
string S;
ll get_hash(ll a,ll b){
if(a > b) return 0;
ll val = hash1[b] - hash1[a-1];
if(val < 0) val += MOD1;
val *= invpot1[a-1];
return val % MOD1;
}
void checa(int pos){
if(pos == N/2 + 1){
if(get_hash(1,pos-1) == get_hash(pos+1,N)){
possiveis.push_back(pos-1);
valores.insert(get_hash(1,pos-1));
}
}
else if(pos <= N/2){
int resta = pos - 1;
ll val1 = get_hash(1,pos - 1);
int falta = N/2 - resta;
ll val2 = get_hash(pos + 1,pos + falta);
ll val3 = (val1 + pot1[resta]*val2) % MOD1;
ll val4 = get_hash(pos+falta+1,N);
if(val3 == val4){
possiveis.push_back(pos-1);
valores.insert(val4);
}
}
else{
int resta = N - pos;
ll val1 = get_hash(pos+1,N);
int falta = N/2 - resta;
ll val2 = get_hash(pos - falta,pos-1);
ll val3 = (val2 + pot1[falta]*val1) % MOD1;
ll val4 = get_hash(1,pos - falta - 1);
if(val3 == val4){
possiveis.push_back(pos-1);
valores.insert(val4);
}
}
}
int main(){
cin >> N >> S;
if(N == 2){
if(S[0] == S[1]) cout << S[0] << endl;
else cout << "NOT UNIQUE" << endl;
return 0;
}
if(N % 2 != 1){
cout << "NOT POSSIBLE" << endl;
return 0;
}
pot1[0] = invpot1[0] = 1;
for(int i = 1;i<=N;i++){
invpot1[i] = (invpot1[i-1]*invprime1) % MOD1;
pot1[i] = (pot1[i-1]*prime1) % MOD1;
hash1[i] = (hash1[i-1] + pot1[i]*S[i-1]) % MOD1;
}
for(int i = 1;i<=N;i++) checa(i);
if(valores.size() == 0){
cout << "NOT POSSIBLE" << endl;
return 0;
}
else if(valores.size() > 1){
cout << "NOT UNIQUE" << endl;
return 0;
}
else{
S.erase(S.begin() + possiveis[0]);
S.resize(N/2);
cout << S << endl;
}
return 0;
}
