#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define pb push_back
const int maxn = 3e5 + 20;
const int mod = 1e9 + 7;
const int base = 4001;
int pw[maxn] , h[maxn];
int get(int l , int r)
{
if(l >= r)
return 0;
return (h[r] - (1LL * h[l] * pw[r - l] % mod) + mod) % mod;
}
int main()
{
ios_base::sync_with_stdio(false);
cin.tie(0);
cout.tie(0);
pw[0] = 1;
for(int i = 1; i < maxn; i++)
pw[i] = 1LL * pw[i - 1] * base % mod;
int n;
string s;
cin >> n >> s;
if(n % 2 == 0)
return cout << "NOT POSSIBLE" << endl , 0;
for(int i = 0; i < n; i++)
h[i + 1] = (1LL * h[i] * base + s[i]) % mod;
int ans = -1 , H = -1;
for(int i = 0; i < n; i++)
{
int h1 , h2;
if(i <= n / 2)
{
h1 = (1LL * get(0 , i) * pw[n / 2 - i] + get(i + 1 , n / 2 + 1)) % mod;
h2 = get(n / 2 + 1 , n);
}
else
{
h1 = get(0 , n / 2);
h2 = (1LL * get(n / 2 , i) * pw[n / 2 - (i - n / 2)] + get(i + 1 , n)) % mod;
}
if(h1 == h2)
{
if(ans != -1 && H != h1)
return cout << "NOT UNIQUE" << endl , 0;
ans = i;
H = h1;
}
}
if(ans == -1)
return cout << "NOT POSSIBLE" << endl , 0;
if(ans <= n / 2)
cout << s.substr(n / 2 + 1 , n / 2) << endl;
else
cout << s.substr(0 , n / 2) << endl;
}
