#include <bits/stdc++.h>
#define int ll
using ll = long long;
int const nmax = 2000001;
int const B = 257;
int const P = 1e9 + 7;
int powerb[1 + nmax];
int pref[2][1 + nmax], suff[2][1 + nmax], solution[1 + nmax];
signed main() {
int n;
std::string s;
std::cin >> n >> s;
if(n % 2 == 0) {
std::cout << "NOT POSSIBLE";
return 0;
}
powerb[0] = 1;
for(int i = 1;i <= nmax; i++)
powerb[i] = 1LL * powerb[i - 1] * B % P;
int half = n / 2;
for(int i = 1;i <= half; i++)
pref[0][i] = (pref[0][i - 1] * B + s[i - 1]) % P;
for(int i = half;0 < i; i--)
suff[0][i] = (suff[0][i + 1] + s[i - 1] * powerb[half - i]) % P;
for(int i = half + 1;i <= n; i++)
pref[1][i] = (pref[1][i - 1] * B + s[i - 1]) % P;
for(int i = n;half < i; i--)
suff[1][i] = (suff[1][i + 1] + s[i - 1] * powerb[n - i]) % P;
int pos = 0;
for(int i = 1;i <= n; i++) {
int prefix = 0;
int suffix = 0;
if(i <= half) {
prefix = (pref[0][i - 1] * powerb[half - i + 1] % P + (suff[0][i + 1] * B + s[half]) % P) % P;
suffix = suff[1][n - half + 1];
} else {
prefix = pref[0][half];
suffix = (pref[1][i - 1] * powerb[n - i] + suff[1][i + 1]) % P;
}
if(prefix == suffix) {
if(pos > 0 && prefix != solution[pos]) {
std::cout << "NOT UNIQUE\n";
return 0;
}
solution[i] = prefix;
pos = i;
}
}
if(pos == 0) {
std::cout << "NOT POSSIBLE";
return 0;
}
if(pos <= half) {
for(int i = n - half;i < n; i++)
std::cout << s[i];
} else {
for(int i = 0;i < half; i++)
std::cout << s[i];
}
return 0;
}
