#include <bits/stdc++.h>
using namespace std;
const int maxn = 2e6+10;
int n;
char num[maxn], a[maxn], b[maxn];
int get_pref(void)
{
int ind = 0;
for (int i = 1; i <= n/2; i++)
{
if (a[i] != b[i]) break;
ind = i;
}
return ind;
}
int get_suf(void)
{
int ind = (n+1)/2+1;
for (int i = (n+1)/2; i >= 2; i--)
{
if (a[i] != b[i-1]) break;
ind = i;
}
return ind;
}
int main(void)
{
ios::sync_with_stdio(false); cin.tie(0);
cin >> n;
for (int i = 1; i <= n; i++)
cin >> num[i];
if (n%2==0)
{
cout << "NOT POSSIBLE\n";
return 0;
}
int ans = 0, add = (n+1)/2, possible = -1;
bool ok = 1;
for (int i = 1; i < add; i++)
if (num[i] != num[i+add]) ok = 0;
if (ok) ans++, possible = add;
for (int i = 1; i <= add; i++)
a[i] = num[i];
for (int i = add+1; i <= n; i++)
b[i-add] = num[i];
int pref = get_pref(), suf = get_suf();
for (int i = 1; i < add; i++)
if (pref >= i-1 && suf <= i+1)
ans++, possible = i;
for (int i = add; i <= n; i++)
a[i-add+1] = num[i];
for (int i = 1; i < add; i++)
b[i] = num[i];
pref = get_pref(), suf = get_suf();
for (int i = add+1; i <= n; i++)
if (pref >= (i-add) && suf <= (i-add)+2)
ans++, possible = i;
if (!ans) cout << "NOT POSSIBLE\n";
else if (ans > 1) cout << "NOT UNIQUE\n";
else
{
for (int i = 1, ind = 1; i <= n && ind < add; i++)
if (i != possible)
cout << num[i], ind++;
cout << "\n";
}
}
