#include <bits/stdc++.h>
using namespace std;
#define addmod(a,b,m) (a + b >= m ? a + b - m : a + b)
typedef long long ll;
const int N = 2000010;
int n;
string s;
ll h[N], p[N], base = 419, mod = 1234567891;
ll get(int l, int r) {
l++, r++;
if(r < l) return 0;
return (h[r] - (p[r-l+1] * h[l-1] % mod) + mod) % mod;
}
int main() {
cin >> n >> s;
if(~n&1) { cout << "NOT POSSIBLE\n"; return 0; }
int len = n/2;
h[0] = 0, p[0] = 1;
for(int i = 0; i < n; i++) {
h[i+1] = (h[i]*base + s[i] - 'A' + 1) % mod;
p[i+1] = p[i] * base % mod;
}
int idx = -1, cnt = 0;
for(int i = 0; i < n; i++) {
int l1 = (i == 0 ? 1 : 0);
int r1 = (l1 < i && i <= l1 + len - 1 ? l1 + len : l1 + len - 1);
ll h1 = get(l1,r1);
int l2 = (r1 + 1 == i ? r1 + 2 : r1 + 1);
int r2 = (i > l2 ? l2 + len : l2 + len - 1);
ll h2 = get(l2,r2);
if(l1 <= i && i <= r1) {
int len = r1 - i;
h1 = get(i+1,r1);
ll part = get(l1,i-1) * p[len] % mod;
h1 = addmod(h1,part,mod);
}
else if(l2 <= i && i <= r2) {
int len = r2 - i;
h2 = get(i+1,r2);
ll part = get(l2,i-1) * p[len] % mod;
h2 = addmod(h2,part,mod);
}
if(h1 == h2) cnt++, idx = i;
}
if(idx == -1) cout << "NOT POSSIBLE\n";
else if(cnt > 1) cout << "NOT UNIQUE\n";
else {
string ans = "";
int now = 0;
for(int i = 0; i < n && now < len; i++) {
if(i == idx) continue;
ans += s[i];
now++;
}
cout << ans << "\n";
}
return 0;
}
