#include <bits/stdc++.h>
using namespace std;
void setup()
{
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
}
int n;
string s;
bool a, b;
inline bool Check(string edited, string base)
{
    string cur = edited;
    cur.erase(cur.begin());
    if (cur == base)
    {
        return true;
    }
    for (int i = 0; i < base.size(); ++i)
    {
        cur[i] = edited[i];
        if (cur == base)
        {
            return true;
        }
    }
    return false;
}
int main()
{
    setup();
    cin >> n >> s;
    if (!(n & 1))
    {
        cout << "NOT POSSIBLE";
        return 0;
    }
    a = Check(s.substr(0, n / 2 + 1), s.substr(n / 2 + 1, n / 2));
    b = Check(s.substr(n / 2, n / 2 + 1), s.substr(0, n / 2));
    if (a && b)
    {
        cout << (s.substr(0, n / 2) == s.substr(n / 2 + 1, n / 2) ? s.substr(0, n / 2) : "NOT UNIQUE");
    }
    else if (a)
    {
        cout << s.substr(n / 2 + 1, n / 2);
    }
    else if (b)
    {
        cout << s.substr(0, n / 2);
    }
    else
    {
        cout << "NOT POSSIBLE";
    }
    return 0;
}
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