#include <bits/stdc++.h>
using namespace std;
const long long INFll = (long long) 1e18 + 10;
const int INFii = (int) 1e9 + 10;
typedef long long ll;
typedef int ii;
typedef long double dbl;
#define endl '\n'
#define sc second
#define fr first
#define mp make_pair
#define pb push_back
#define all(x) x.begin(), x.end()
#define maxn 2000010
ii n, sf[maxn];
string u, s1, s2;
int main() {
ios::sync_with_stdio(false); cin.tie(0);
//freopen("in.in", "r", stdin);
cin >> n >> u;
if(n%2 == 0) {
cout << "NOT POSSIBLE" << endl;
return 0;
}
ii mid = n/2;
//cout << u[mid] << endl;
string ans1, ans2;
for(ii i = 0; i < mid; i++) {
s1.pb(u[i]);
}
for(ii i = mid; i < n; i++) {
s2.pb(u[i]);
}
//cout << mid << " " << s1 << " " << s2 << endl;
sf[mid] = 1;
for(ii i = mid-1; i >= 0; i--) {
//cout << s1[i] << " " << s2[i+1] << endl;
if(s1[i] == s2[i+1]) {
sf[i] = 1;
}
else {
sf[i] = 0;
}
if(i != mid) sf[i] = min(sf[i], sf[i+1]);
}
for(ii i = 0; i <= mid; i++) {
//cout << i << " pf " << pf[i] << endl;
//cout << i+1 << " sf " << sf[i+1] << endl;
//cout << endl;
if(sf[i] == 1) {
if(ans1.size() == 0) ans1 = s1;
}
if(s1[i] != s2[i]) break;
}
//segunda parte
s1.clear();
s2.clear();
for(ii i = 0; i <= mid; i++) {
s2.pb(u[i]);
}
for(ii i = mid+1; i < n; i++) {
s1.pb(u[i]);
}
//cout << mid << " " << s1 << " " << s2 << endl;
sf[mid] = 1;
for(ii i = mid-1; i >= 0; i--) {
//cout << s1[i] << " " << s2[i+1] << endl;
if(s1[i] == s2[i+1]) {
sf[i] = 1;
}
else {
sf[i] = 0;
}
if(i != mid) sf[i] = min(sf[i], sf[i+1]);
}
for(ii i = 0; i <= mid; i++) {
//cout << i << " pf " << pf[i] << endl;
//cout << i+1 << " sf " << sf[i+1] << endl;
//cout << endl;
if(sf[i] == 1) {
if(ans2.size() == 0) ans2 = s1;
}
if(s1[i] != s2[i]) break;
}
if(ans1 == ans2) {
ans2 = "";
}
if(ans1.size() < ans2.size()) swap(ans1,ans2);
if(ans1.size() == 0)
cout << "NOT POSSIBLE" << endl;
else if(ans2.size() == 0)
cout << ans1 << endl;
else {
cout << "NOT UNIQUE" << endl;
}
return 0;
}
