// #pragma GCC optimize("O3,unroll-loops")
// #pragma GCC target("avx2,bmi,bmi2,lzcnt,popcnt")
#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define pii pair<int, int>
#define pll pair<ll, ll>
#define vi vector<int>
#define vl vector<ll>
#define vii vector<pii>
#define db long double
#define vll vector<pll>
#define endl '\n'
#define all(x) x.begin(), x.end()
#define fastio ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
#define int long long
const int sz = 2e5 + 5;
const int mod = 1e9 + 7;
int pw[sz], inv[sz], pre[sz], n;
string s;
int sum(int a, int b){
return (a + b) % mod;
}
int mult(int a, int b){
return a % mod * (b % mod) % mod;
}
int sub(int a, int b){
return (a + mod - b) % mod;
}
int binpow(int a, int b){
if(b == 0) return 1;
if(b & 1) return (a % mod * binpow(a, b - 1)) % mod;
return binpow(a * a % mod, b / 2) % mod;
}
int get_hash(int l, int r){
return mult(sub(pre[r], pre[l - 1]), inv[l - 1]);
}
void fmain(){
cin >> n >> s;
s = ' ' + s;
pw[0] = inv[0] = 1;
for(int i = 1; i <= n; i++){
pw[i] = mult(pw[i - 1], 29);
inv[i] = binpow(pw[i], mod - 2);
pre[i] = sum(pre[i - 1], mult(s[i] - 'A' + 1, pw[i]));
}
set<int> st;
int k = n / 2, lst = -1;
for(int i = 1; i <= n; i++){
int s1 = get_hash(1, i - 1), s2 = get_hash(i + 1, n);
// cout << "before : " << s1 << " " << s2 << endl;
if(i <= k){
int m = k - i + 1;
s1 = sum(s1, mult(get_hash(i + 1, i + m), pw[i - 1]));
s2 = mult(sub(s2, get_hash(i + 1, i + m)), inv[m]);
}
else{
int m = k - (n - i);
s1 = sub(s1, mult(get_hash(i - m, i - 1), pw[i - m - 1]));
s2 = sum(get_hash(i - m, i - 1), mult(s2, pw[m]));
}
if(s1 == s2) st.insert(s1), lst = i;
// cout << "After : " << s1 << " " << s2 << endl;
}
if(st.empty()) cout << "NOT POSSIBLE";
else if(st.size() > 1) cout << "NOT UNIQUE";
else{
// cout << lst << endl;
string ans = "";
if(lst <= k){
for(int i = 1; i < lst; i++)ans += s[i];
for(int i = 1; i <= k - lst + 1; i++) ans += s[lst + i];
}
else{
for(int i = 1; i <= k; i++){
ans += s[i];
}
for(int i = k + 1; i <= n; i++){
if(i == lst) continue;
ans += s[i];
}
}
cout << ans;
}
// 1, 2 i = 3 4, 5, 6, 7, 8, 9, 10, 11
}
signed main(){
int tmr = 1;
// cin >> tmr;
while(tmr--){
fmain();
}
}
