#include <bits/stdc++.h>
using namespace std;
const int maxn = 2e6+10;
int n;
char num[maxn];
vector<char> a, b;
int get_pref(void)
{
int ind = 0;
for (int i = 0; i < n/2; i++)
{
if (a[i] != b[i]) break;
ind = i+1;
}
return ind;
}
int get_suf(void)
{
int ind = (n+1)/2+1;
for (int i = (n+1)/2-1; i >= 1; i--)
{
if (a[i] != b[i-1]) break;
ind = i+1;
}
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 add = (n+1)/2;
bool ok = 1;
for (int i = 1; i < add; i++)
if (num[i] != num[i+add])
ok = 0;
if (ok)
{
for (int i = 1; i < add; i++)
cout << num[i];
cout << "\n";
return 0;
}
bool deu1 = 0, deu2 = 0;
for (int i = 1; i <= add; i++)
a.push_back(num[i]);
for (int i = add+1; i <= n; i++)
b.push_back(num[i]);
int pref = get_pref(), suf = get_suf();
for (int i = 1; i < add; i++)
if (pref >= i-1 && suf <= i+1)
deu1 = 1;
a.clear(), b.clear();
for (int i = add; i <= n; i++)
a.push_back(num[i]);
for (int i = 1; i < add; i++)
b.push_back(num[i]);
pref = get_pref(), suf = get_suf();
for (int i = add+1; i <= n; i++)
if (pref >= (i-add) && suf <= (i-add)+2)
deu2 = 1;
if (!deu1 && !deu2) cout << "NOT POSSIBLE\n";
else if (deu1 && deu2) cout << "NOT UNIQUE\n";
else
{
if (deu1)
for (int i = add+1; i <= n; i++)
cout << num[i];
else
for (int i = 1; i < add; i++)
cout << num[i];
cout << "\n";
}
}
