#include <bits/stdc++.h>
using namespace std;
const int N = 2000006;
int n;
char aa[N], a[N];
int pp, ss;
int main()
{
//freopen("input.txt", "r", stdin);
cin >> n >> aa;
for (int i = 1; i <= n; ++i)
a[i] = aa[i - 1];
if (n % 2 == 0)
{
cout << "NOT POSSIBLE" << endl;
return 0;
}
for (int i = 1, j = n / 2 + 2; i <= n / 2; ++i, ++j)
{
if (a[i] != a[j])
break;
++pp;
}
for (int i = n / 2 + 1, j = n; j > n / 2 + 1; --i, --j)
{
if (a[i] != a[j])
break;
++ss;
}
bool z2 = false;
for (int i = 1; i <= n / 2; ++i)
{
if (pp >= i - 1 && ss >= (n / 2 - i + 1))
{
z2 = true;
break;
}
}
pp = ss = 0;
for (int i = 1, j = n / 2 + 1; i <= n / 2; ++i, ++j)
{
if (a[i] != a[j])
break;
++pp;
}
for (int i = n / 2, j = n; i >= 1; --i, --j)
{
if (a[i] != a[j])
break;
++ss;
}
bool z1 = false;
for (int i = n / 2 + 2; i <= n; ++i)
{
if (ss >= (n - i) && pp >= (i - n / 2 - 1))
{
z1 = true;
break;
}
}
bool z = true;
for (int i = 1, j = n / 2 + 2; i <= n / 2; ++i, ++j)
{
if (a[i] != a[j])
{
z = false;
break;
}
}
if (!z && !z1 && !z2)
{
cout << "NOT POSSIBLE" << endl;
return 0;
}
if (z1 && z2 && !z)
{
cout << "NOT UNIQUE" << endl;
return 0;
}
if (z1 || z)
{
for (int i = 1; i <= n / 2; ++i)
{
cout << a[i];
}
cout << endl;
return 0;
}
if (z2)
{
for (int i = n / 2 + 2; i <= n; ++i)
{
cout << a[i];
}
cout << endl;
return 0;
}
return 0;
}
