#include <bits/stdc++.h>
#pragma GCC optimize("O3")
#define FOR(i, x, y) for(int i = x; i < y; i++)
typedef long long ll;
using namespace std;
int main() {
ios_base::sync_with_stdio(false);
cin.tie(NULL);
ll n;
cin >> n;
if (n % 2 == 0) return cout << "IMPOSSIBLE\n", 0;
string s;
cin >> s;
string a = s.substr(0, n / 2), b = s.substr(n / 2 + 1, n / 2);
if (a == b) return cout << a << '\n', 0;
bool a_possible = true, b_possible = true;
bool chucked = false;
ll pos = 0;
FOR(i, n / 2, n) {
if (pos == n / 2) break;
else if (a[pos] == s[i]) pos++;
else if (!chucked) chucked = true;
else {
a_possible = false;
break;
}
}
chucked = false;
pos = 0;
FOR(i, 0, n / 2 + 1) {
if (pos == n / 2) break;
else if (b[pos] == s[i]) pos++;
else if (!chucked) chucked = true;
else {
b_possible = false;
break;
}
}
if (a_possible && b_possible) cout << "NOT UNIQUE\n";
else if (a_possible) cout << a << '\n';
else if (b_possible) cout << b << '\n';
else cout << "IMPOSSIBLE\n";
return 0;
}
