#include <bits/stdc++.h>
using namespace std;
#define int ll
#define endl '\n' //comment for interactive
#define fast_io ios::sync_with_stdio(0); cin.tie(0); cout.tie(0)
#define pb push_back
#define re resize
#define ff first
#define ss second
#define all(x) (x).begin(), (x).end()
#define loop(i, n) for(int i = 0; i < n; i++)
#define loop1(i, n) for(int i = 1; i <= n; i++)
#define print(x) cout << #x << ": " << x << endl << flush
typedef long long ll;
typedef vector<int> vi;
typedef array<int, 2> ii;
typedef array<int, 3> ti;
typedef vector<ii> vii;
typedef vector <ti> vti;
typedef priority_queue<int> pq;
template<class T> bool ckmin(T&a, T b) { bool B = a > b; a = min(a, b); return B; }
template<class T> bool ckmax(T&a, T b) { bool B = a < b; a = max(a, b); return B; }
const int inf = 1e17;
//const int maxn = ;
string check(string a, string b) {
// a smol b big
int n = a.size();
bool ok = true;
char c = ' ';
int i = 0, j = 0;
while(i < n) {
if(a[i] != b[j]) {
if(c != ' ') {
ok = false;
break;
} else {
c = b[j];
j++;
}
} else {
i++; j++;
}
}
if(ok) return a;
else return "";
}
void solve() {
int n; cin >> n;
string s; cin >> s;
if(n <= 2 or n % 2 == 0) {
cout << "NOT POSSIBLE" << endl;
return;
}
string a1 = check(s.substr(0, n/2), s.substr(n/2, n/2 + 1));
string a2 = check(s.substr(n/2 + 1, n/2), s.substr(0, n/2 + 1));
if(a1 == "" and a2 == "")
cout << "NOT POSSIBLE" << endl;
else if(a1 == "")
cout << a2 << endl;
else if(a2 == "")
cout << a1 << endl;
else
cout << "NOT UNIQUE" << endl;
}
signed main() {
fast_io;
int t = 1; //cin >> t;
while(t--)
solve();
return 0;
}
