//: // ////: /// / : //// / :
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<ll, ll> pll;
#define all(x) (x).begin(),(x).end()
#define X first
#define Y second
#define sep ' '
#define endl '\n'
#define debug(x) cerr << #x << ": " << x << endl;
const ll MAXN = 3e6 + 10;
const ll P = 78;
const ll MOD = 1e9 + 7;
int n, ans = MAXN;
ll pw[MAXN], h1, h2, suff[MAXN];
string s;
inline ll hsh(string s) {
ll ans = 0;
for (char c : s)
ans = (ans * P + int(c)) % MOD;
return ans;
}
int main() {
ios_base::sync_with_stdio(false); cin.tie(0); cout.tie(0);
cin >> n >> s;
if (!(n & 1)) return cout << "NOT POSSIBLE" << endl, 0;
string t;
for (int i = 0; i < n / 2; i++) t.push_back(s[i]);
h1 = hsh(t + t);
t.clear();
for (int i = n - 1; i > n - 1 - n / 2; i--)
t.push_back(s[i]);
reverse(all(t));
h2 = hsh(t + t);
pw[0] = 1;
for (int i = 1; i < n + 10; i++)
pw[i] = pw[i - 1] * P % MOD;
for (int i = n - 1; i >= 0; i--)
suff[i] = (suff[i + 1] + s[i] * pw[n - 1 - i]) % MOD;
ll h = 0;
for (int i = 0; i < n; i++) {
ll th = (h * pw[n - i - 1] + suff[i + 1]) % MOD;
if (th == h1 || th == h2) {
if (ans == MAXN) ans = i;
else return cout << "NOT UNIQUE" << endl, 0;
}
h = (h * P + s[i]) % MOD;
}
if (ans == MAXN) return cout << "NOT POSSIBLE" << endl, 0;
s.erase(s.begin() + ans);
for (int i = 0; i < n / 2; i++) s.pop_back();
cout << s << endl;
return 0;
}
