/*
ID: agageld1
LANG: C++17
TASK:
*/
#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define N 400005
#define ff first
#define ss second
#define pb push_back
#define sz(s) (int)s.size()
#define rep(c, a, b) for(c = a; c <= b; c++)
//mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
ll n, t, a[N];
string s, g;
vector <string> v, h;
int main () {
ios::sync_with_stdio(0);cin.tie(0);
cin >> n >> s;
if(n%2 == 0 || n == 1) {
cout << "NOT POSSIBLE";
return 0;
}
for(int i = 0; i < n; i++) {
g = "";
v.clear();
for(int j = 0;j<n;j++) {
if(j == i) continue;
g += s[j];
if(sz(g) == (n / 2)) {
v.pb(g);
g = "";
}
}
assert(sz(v[0]) == sz(v[1]));
if(v[0] == v[1]) {
h.pb(v[0]);
}
}
if(!sz(h)) {
cout << "NOT POSSIBLE";
return 0;
}
if(sz(h) > 1) {
cout << "NOT UNIQUE";
return 0;
}
cout << h[0];
}
