/*
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], p = 0;
string s, g, ans;
void solve(string h) {
int cnt = 0, tr = 0;
for(int i = 0; i < sz(s); i++) {
if(cnt == sz(h)) break;
if(h[cnt] == s[i]) cnt++;
else {
tr++;
if(tr == 2) break;
}
}
if(tr <= 1) p = 1;
}
int main () {
ios::sync_with_stdio(0);cin.tie(0);
cin >> n >> s;
if(n % 2 == 0) {
cout << "NOT POSSIBLE";
return 0;
}
for(int i = 0; i < n/2; i++) {
g += s[i];
}
int f = 0;
solve(g + g);
if(p) {
ans = g;
f++;
}
g = "";
for(int i = n/2 + 1; i < n; i++) {
g += s[i];
}
p = 0;
solve(g + g);
if(p) {
if(sz(ans) == 0 || ans != g){
ans = g;
f++;
}
}
if(!f) cout << "NOT POSSIBLE\n";
if(f == 2) cout << "NOT UNIQUE\n";
else cout << ans << '\n';
}
