#include <bits/stdc++.h>
using namespace std;
#pragma GCC optimize("Ofast,inline") // Ofast = O3,fast-math,allow-store-data-races,no-protect-parens
#pragma GCC optimize ("unroll-loops")
#pragma GCC target("bmi,bmi2,lzcnt,popcnt") // bit manipulation
#pragma GCC target("movbe") // byte swap
#pragma GCC target("aes,pclmul,rdrnd") // encryption
#pragma GCC target("avx,avx2,f16c,fma,sse3,ssse3,sse4.1,sse4.2")
typedef long long ll;
#define rep(i, a, b) for (int i = (a); i < int(b); i++)
typedef pair<ll, ll> pll;
typedef pair<int, int> pii;
#define pb push_back
inline void fast() { ios::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL); }
ll INF = 1000000000;
ll mod = 1e9 + 7;
#define int ll
#define float double
//
ll N;
string s;
int32_t main()
{
fast();
cin >> N;
cin >> s;
if (N % 2 == 0) {
cout << "NOT POSSIBLE\n";
exit(0);
}
ll n = N / 2;
vector<string> ans;
if (s[1] == s[n + 1]) {
string ns = "";
ns.pb(s[1]);
rep(at, 2, n + 1) {
if (s[at] == s[n + at]) {
ns.pb(s[at]);
}
else break;
}
if (ns.size() == n) {
ans.pb(ns);
}
}
if (s[0] == s[n]) {
int p = 0;
string ns = "";
ns.pb(s[0]);
rep(at, 1, n) {
if (s[at] == s[n + at + p]) {
ns.pb(s[at]);
}
else if (p == 0) {
p = 1;
at--;
}
else break;
}
if (ns.size() == n) {
if (ans.size() == 1) {
bool same = true;
rep(i,0,n) {
if (ns[i] != ans[0][i]) {
same = false;
break;
}
}
if (!same) ans.pb(ns);
}
else ans.pb(ns);
}
}
if (s[0] == s[n+1]) {
int p = 0;
string ns = "";
ns.pb(s[n+1]);
rep(at, 1, n) {
if (s[at + p] == s[n + 1 + at]) {
ns.pb(s[n + 1 + at]);
}
else if (p == 0) {
p = 1;
at--;
}
else break;
}
if (ns.size() == n) {
if (ans.size() > 0) {
bool bb = false;
rep(j, 0, ans.size()) {
bool same = true;
rep(i,0,n) {
if (ns[i] != ans[j][i]) {
same = false;
break;
}
}
if (same) bb = true;
}
if (!bb) ans.pb(ns);
}
else ans.pb(ns);
}
}
if (ans.size() == 1) cout << ans[0] << "\n";
else if (ans.size() > 1) cout << "NOT UNIQUE\n";
else cout << "NOT POSSIBLE\n";
}
