#include <bits/stdc++.h>
using namespace std;
int main() {
ios::sync_with_stdio(false);
cin.tie(NULL);
int N; cin >> N;
string S; cin >> S;
if (N % 2 == 0) {
cout << "NOT POSSIBLE\n";
return 0;
}
int extraChar = 0, numSubstr = 0;
char left = ' ', right = ' ';
for (int i = 1; i <= N/2; i++)
left ^= S[i];
for (int i = N/2 + 1; i < N; i++)
right ^= S[i];
// first char
if (left == right) numSubstr++;
// left characters
for (int currChar = 1; currChar <= N/2; currChar++) {
left ^= S[currChar];
left ^= S[currChar - 1];
if (left == right) {
numSubstr++;
extraChar = currChar;
}
}
// right characters
for (int currChar = N/2 + 1; currChar < N; currChar++) {
right ^= S[currChar];
right ^= S[currChar - 1];
if (left == right) {
numSubstr++;
extraChar = currChar;
}
}
if (numSubstr == 0) {
cout << "NOT POSSIBLE\n";
} else if (numSubstr > 1) {
cout << "NOT UNIQUE\n";
} else {
int i = 0;
while (i < N/2) {
if (i != extraChar) cout << S[i];
i++;
}
if (extraChar <= N/2) cout << S[i];
cout << '\n';
}
return 0;
}
