#include <bits/stdc++.h>
using namespace std;
int main() {
ios::sync_with_stdio(false);
cin.tie(NULL); cout.tie(NULL);
int n; cin >> n;
string s; cin >> s;
if (n % 2 == 0) {
cout << "NOT POSSIBLE\n";
return 0;
}
// left:
bool left = true, l = true;
int a = 0, b = n/2;
string leftPart = "";
while (a < n/2 && b < n) {
if (s[a] != s[b]) {
if (l) {
b++;
l = false;
} else {
left = false;
}
}
leftPart += s[a];
a++; b++;
}
// right:
bool right = true, r = true;
int c = (n+1)/2, d = 0;
string rightPart = "";
while (c < n && d < (n+1)/2) {
if (s[c] != s[d]) {
if (r) {
d++;
r = false;
} else {
right = false;
}
}
rightPart += s[c];
c++; d++;
}
if (left && right) {
if (leftPart == rightPart) cout << leftPart << '\n';
else cout << "NOT UNIQUE\n";
} else if (left) {
cout << leftPart << '\n';
} else if (right) {
cout << rightPart << '\n';
} else {
cout << "NOT POSSIBLE\n";
}
return 0;
}
