#include "bits/stdc++.h"
using namespace std;
#ifdef duc_debug
#include "bits/debug.h"
#else
#define debug(...)
#endif
const int maxn = 2e6 + 5;
const int mod[] = {1000000009, 141257};
const int base = 311;
int n;
string s;
int h[maxn][2], p[maxn][2];
void solve() {
cin >> n >> s;
if (n % 2 == 0) {
cout << "NOT POSSIBLE\n";
return;
}
s = ' ' + s;
for (int c = 0; c < 2; ++c) {
for (int i = 1; i <= n; ++i) {
h[i][c] = ((1ll * h[i - 1][c] * base % mod[c]) + (s[i] - 'A' + 1)) % mod[c];
}
}
auto get_hash = [&](int l, int r) {
if (l > r) return make_pair(0, 0);
int x[2];
for (int c = 0; c < 2; ++c) {
x[c] = (h[r][c] - (1ll * h[l - 1][c] * p[r - l + 1][c] % mod[c]) + mod[c]) % mod[c];
}
return make_pair(x[0], x[1]);
};
vector<int> pos;
vector<pair<int, int>> hc;
pair<int, int> ft = get_hash(n / 2 + 2, n);
pair<int, int> se = make_pair(h[n / 2][0], h[n / 2][1]);
for (int i = 1; i <= n; ++i) {
pair<int, int> hh = (i <= n / 2 ? ft : se);
pair<int, int> x = get_hash(1, i - 1), y = get_hash(i + 1, n);
pair<int, int> rm;
rm.first = (1ll * x.first * p[n - i][0] % mod[0] + y.first) % mod[0];
rm.second = (1ll * x.second * p[n - i][1] % mod[1] + y.second) % mod[1];
hh.first = (1ll * hh.first * p[n / 2][0] % mod[0] + hh.first) % mod[0];
hh.second = (1ll * hh.second * p[n / 2][1] % mod[1] + hh.second) % mod[1];
if (rm == hh) {
pos.push_back(i);
hc.push_back(hh);
}
}
sort(hc.begin(), hc.end());
hc.erase(unique(hc.begin(), hc.end()), hc.end());
if (pos.empty()) {
cout << "NOT POSSIBLE\n";
} else if ((int)hc.size() > 1) {
cout << "NOT UNIQUE\n";
} else {
s.erase(s.begin() + pos[0]);
for (int i = 1; i <= n / 2; ++i) {
cout << s[i];
}
}
}
signed main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
for (int c = 0; c < 2; ++c) {
p[0][c] = 1;
for (int i = 1; i < maxn; ++i) {
p[i][c] = 1ll * p[i - 1][c] * base % mod[c];
}
}
solve();
return 0;
}
