#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const ll mod = 1000000007;
const ll p = 31;
const ll pinv = 129032259;
const int MAXN = 2000010;
int N, ans1, ans2;
int S[MAXN];
ll h[MAXN];
ll ppow[MAXN];
ll ppowinv[MAXN];
ll subhash(int a, int b) {
if (a > b) return 0;
ll res = h[b] - h[a-1];
if (res < 0) res += mod;
res = (res * ppowinv[a]) % mod;
return res;
}
ll combine(int a, int b, int c, int d) {
ll x = subhash(a, b);
ll y = subhash(c, d);
y = (y * ppow[b-a+1]) % mod;
return (x + y) % mod;
}
int main() {
ios::sync_with_stdio(false); cin.tie(0);
cin >> N;
if (N % 2 == 0) {
cout << "NOT POSSIBLE\n";
return 0;
}
for (int i = 1; i <= N; ++i) {
char ch;
cin >> ch;
S[i] = ch - 'A' + 1;
}
ppow[0] = ppowinv[0] = 1;
for (int i = 1; i <= N; ++i) {
ppow[i] = (ppow[i-1] * p) % mod;
ppowinv[i] = (ppowinv[i-1] * pinv) % mod;
}
h[0] = 0;
for (int i = 1; i <= N; ++i) {
h[i] = (h[i-1] + ppow[i]*S[i]) % mod;
}
ans1 = -1;
for (int i = 1; i <= N; ++i) {
if (i <= N/2) {
if (combine(1, i-1, i+1, (N/2)+1) == subhash((N/2)+2, N)) {
if (ans1 != -1) {
cout << "NOT UNIQUE\n";
return 0;
}
ans1 = (N/2)+2;
ans2 = N;
}
} else if (i == (N/2)+1) {
if (subhash(1, i-1) == subhash(i+1, N)) {
if (ans1 != -1) {
cout << "NOT UNIQUE\n";
return 0;
}
ans1 = 1;
ans2 = N/2;
}
} else {
if (subhash(1, N/2) == combine((N/2)+1, i-1, i+1, N)) {
if (ans1 != -1) {
cout << "NOT UNIQUE\n";
return 0;
}
ans1 = 1;
ans2 = N/2;
}
}
}
if (ans1 == -1) {
cout << "NOT POSSIBLE\n";
} else {
for (int i = ans1; i <= ans2; ++i) {
cout << char(S[i] + 'A' - 1);
}
cout << "\n";
}
}
