#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const string not_possible = "NOT POSSIBLE";
const string not_unique = "NOT UNIQUE";
const ll MOD = 1645333507;
const ll BASE = 137;
int n, m;
string s;
ll mod(ll x) {
x %= MOD;
if (x < 0)
return x + MOD;
else
return x;
}
ll append(ll prev, char a) { return mod(prev * BASE + ll(a - 'A')); }
ll fastpow(ll x, ll pot) {
ll result = 1;
ll current = x;
while (pot != 0) {
if (pot & 1) result = mod(result * current);
current = mod(current * current);
pot >>= 1;
}
return result;
}
vector<ll> h;
void buildHash() {
h.resize(n);
for (int i = 0; i < n; i++) {
h[i] = append(i > 0 ? h[i - 1] : 0, s[i]);
}
}
ll getHash(int i, int j) {
ll result = mod(h[j] - (i > 0 ? h[i - 1] : 0) * fastpow(BASE, j - i + 1));
return result;
}
ll getHash(int a, int b, int c, int d) {
ll result = mod(getHash(a, b) * fastpow(BASE, d - c + 1) + getHash(c, d));
return result;
}
int main() {
cin >> n >> s;
m = n / 2;
if (n < 3 || n != m * 2 + 1) {
cout << not_possible << endl;
return 0;
}
buildHash();
return 0;
map<ll, int> ans;
if (getHash(1, m) == getHash(m + 1, n - 1)) ans[getHash(1, m)] = 0;
if (getHash(0, m - 1) == getHash(m + 1, n - 1)) ans[getHash(0, m - 1)] = m;
if (getHash(0, m - 1) == getHash(m, n - 2)) ans[getHash(0, m - 1)] = n - 1;
for (int i = 1; i < m; i++) {
if (getHash(0, i - 1, i + 1, m) == getHash(m + 1, n - 1))
ans[getHash(m + 1, n - 1)] = i;
}
for (int i = m + 1; i < n - 1; i++) {
if (getHash(0, m - 1) == getHash(m, i - 1, i + 1, n - 1))
ans[getHash(0, m - 1)] = i;
}
if (ans.size() == 0)
cout << not_possible << endl;
else if (ans.size() > 1)
cout << not_unique << endl;
else
cout << s.substr(ans.begin()->second < m ? m + 1 : 0, m) << endl;
}
