#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<ll, ll> pll;
#define fastio ios::sync_with_stdio(false), cin.tie(0)
#pragma GCC optimize("Ofast")
#define pb push_back
#define eb emplace_back
#define f first
#define s second
#define int long long
#define lowbit(x) x&-x
const int INF = 1e9 + 7;
const int maxn = 2e6 + 5;
const int A = 923984352;
const int N = 998244353;
int p[maxn];
signed main(void){
fastio;
int n;
cin>>n;
p[0] = 1;
for(int i = 1; i < n; i++) p[i] = p[i - 1] * A % N;
string s;
cin>>s;
if(n % 2 == 0){
cout<<"NOT POSSIBLE\n";
return 0;
}
vector<int> h(n);
h[0] = s[0];
for(int i = 1; i < n; i++){
h[i] = (h[i - 1] * A + s[i]) % N;
}
int len = n / 2;
set<int> match;
int ans = 0;
for(int i = 0; i < n; i++){
if(i <= len){
int a = ((i ? h[i - 1] * p[len - i] : 0) + h[len] - h[i] * p[len - i] % N + N) % N;
int b = (h[n - 1] - h[len] * p[len] % N + N) % N;
if(a == b) match.insert(a), ans = i;
}
else{
int a = h[len - 1];
int b = ((h[n - 1] - h[i] * p[n - 1 - i] % N + N) + (h[i - 1] - h[len - 1] * p[i - len]) % N * p[n - 1 - i] % N + 2 * N) % N;
if(a == b) match.insert(a), ans = i;
}
}
if(match.size() >= 2){
cout<<"NOT UNIQUE\n";
}
else if(!match.size()){
cout<<"NOT POSSIBLE\n";
}
else{
if(ans >= len) cout<<s.substr(0, len);
else cout<<s.substr(len + 1, len);
cout<<"\n";
}
}
