#include <iostream>
#include <cstring>
using namespace std;
bool isSame(string s1, string s2)
{
for (int i = 0; i < signed(s1.size()); i++)
{
if (s1[i] != s2[i])
return false;
}
return true;
}
int main()
{
int n;
cin >> n;
/*
while (true)
{
string t;
for (int i = 0; i < n - 1; i++)
t += char((rand() % 26) + 65);
cout << t << endl;
cout << t.substr(0, int((n - 1) / 2)) << endl;
cout << t.substr(int((n - 1) / 2), n - 1) << endl;
cin >> t;
}*/
if (n % 2 == 0)
{
cout << "NOT POSSIBLE" << endl;
return 0;
}
string s;
string res;
int resNum = 0;
cin >> s;
for (int i = 0; i < n; i++)
{
string t = s;
for (int j = i; j < n; j++)
{
t[j] = t[j + 1];
}
/*
for (int j = 0; j < n - 1; j++)
cout << t[j];
cout << endl;*/
/*
cout << t.substr(0, int((n - 1) / 2)) << endl;
cout << t.substr(int((n - 1) / 2), n - 1) << endl;*/
if (isSame(t.substr(0, int((n - 1) / 2)), t.substr(int((n - 1) / 2), n - 1)))
{
resNum++;
res = t.substr(0, int((n - 1) / 2));
}
}
if (resNum == 0)
cout << "NOT POSSIBLE" << endl;
else if (resNum > 1)
cout << "NOT UNIQUE" << endl;
else
cout << res << endl;
return 0;
}
