// header file
#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
// pragma
#pragma GCC optimize("Ofast")
#pragma GCC optimize("unroll-loops")
// macros
#define endl "\n"
#define ll long long
#define mp make_pair
#define ins insert
#define lb lower_bound
#define pb push_back
#define ub upper_bound
#define lll __int128
#define fi first
#define se second
using namespace std;
using namespace __gnu_pbds;
typedef tree<int, null_type, less_equal<int>, rb_tree_tag, tree_order_statistics_node_update> ordered_multiset;
typedef tree<int, null_type, less<int>, rb_tree_tag,tree_order_statistics_node_update> ordered_set;
typedef unsigned long long ull;
ull modmul(ull a, ull b, ull M) {
ll ret = a * b - M * ull(1.L / M * a * b);
return ret + M * (ret < 0) - M * (ret >= (ll)M);
}
struct shash {
ull mod;
vector<ull> pref, power;
shash(string &s, ull key, ull m) {
mod = m;
pref = power = vector<ull>(s.size() + 1, 0);
power[0] = 1;
for(int i = 0; i < s.size(); ++i) {
pref[i + 1] = modmul(pref[i], key, mod) + s[i];
pref[i + 1] = pref[i + 1] >= mod ? pref[i + 1] - mod : pref[i + 1];
power[i + 1] = modmul(power[i], key, mod);
}
}
ull norm(ll a) {
if(a < 0)
return a + mod;
else if(a >= mod)
return a - mod;
else
return a;
}
ull query(int l, int r) {
return norm((ll)pref[r] - modmul(pref[l - 1], power[r - l + 1], mod));
}
ull query(int l1, int r1, int l2, int r2) {
return norm(modmul(query(l1, r1), power[r2 - l2 + 1], mod) + query(l2, r2));
}
};
int main() {
ios_base::sync_with_stdio(0); cin.tie(NULL);
int n;
cin >> n;
string s;
cin >> s;
if(!(n & 1)) {
cout << "NOT POSSIBLE" << endl;
exit(0);
}
mt19937 rng(chrono::steady_clock().now().time_since_epoch().count());
const ull mod = (1ll << 61) - 1;
shash h(s, rng() % mod, mod);
int cnt = 0, idx = -1;
ll pr = -1;
for(int i = 1; i <= n; ++i) {
// without the i-th index, is the rest equal?
int old_cnt = cnt;
if(i <= n / 2) {
if(h.query(1, i - 1, i + 1, n / 2 + 1) == h.query(n / 2 + 2, n))
cnt += h.query(n / 2 + 2, n) != pr, pr = h.query(n / 2 + 2, n);
}
else if(i == n / 2 + 1) {
if(h.query(1, i - 1) == h.query(i + 1, n))
cnt += h.query(1, i - 1) != pr, pr = h.query(1, i - 1);
}
else {
if(h.query(1, n / 2) == h.query(n / 2 + 1, i - 1, i + 1, n))
cnt += h.query(1, n / 2) != pr, pr = h.query(1, n / 2);
}
idx = old_cnt == cnt ? idx : i;
}
if(cnt == 1) {
int out = 0;
for(int i = 0; i < n && out < n / 2; ++i) {
if(i == idx - 1)
continue;
cout << s[i], ++out;
}
cout << endl;
}
else
cout << (cnt == 0 ? "NOT POSSIBLE" : "NOT UNIQUE") << endl;
return 0;
}
