#include <iostream>
using namespace std;
#define int long long
int h[2<<20], pw[2<<20], mod = 998244353;
int get(int l, int r){
return (h[r] - h[--l] * pw[r - l] % mod + mod) % mod;
}
signed main(){
ios_base::sync_with_stdio(false); cin.tie(NULL);
int n, id = -1, Vl = 0;
string s;
cin>>n>>s;
for (int i=pw[0]=1;i<=n;i++){
pw[i] = pw[i-1] * 256 % mod;
h[i] = (h[i-1] * 256 + s[i-1]) % mod;
}
for (int i=1;i<=n and n % 2;i++){
int h1, h2;
if (i <= n / 2){
h1 = (get(1, i-1) * pw[n / 2 - (i - 1)] + get(i + 1, n / 2 + 1)) % mod;
h2 = get(n / 2 + 2, n);
}
else{
h1 = get(1, n / 2);
h2 = (get(n / 2 + 1, i - 1) * pw[n - i] + get(i + 1, n)) % mod;
}
if (h1 == h2 and id == -1)
id = i, Vl = h1;
else if (h1 == h2 and Vl != h1)
return cout<<"NOT UNIQUE\n", 0;
}
if (id == -1)
return cout<<"NOT POSSIBLE\n", 0;
for (int i=1;i<=n / 2;i++){
if (i == id)
n += 2;
else
cout<<s[i-1];
}
cout<<'\n';
}
