#include <bits/stdc++.h>
using namespace std;
const int maxn = 1e5+10;
int n;
char num[maxn], a[maxn], b[maxn];
bool ok_pref(int pos)
{
for (int i = 1; i <= pos; i++)
if (a[i] != b[i])
return false;
return true;
}
bool ok_suf(int pos)
{
for (int i = (n+1)/2; i >= pos; i--)
if (a[i] != b[i-1])
return false;
return true;
}
int busca_pref(void)
{
int ini = 1, fim = n/2, ans = 0;
while (ini <= fim)
{
int mid = (ini+fim)>>1;
if (ok_pref(mid)) ans = mid, ini = mid+1;
else fim = mid-1;
}
return ans;
}
int busca_suf(void)
{
int ini = 2, fim = (n+1)/2, ans = fim+1;
while (ini <= fim)
{
int mid = (ini+fim)>>1;
if (ok_suf(mid)) ans = mid, fim = mid-1;
else ini = mid+1;
}
return ans;
}
int main(void)
{
ios::sync_with_stdio(false); cin.tie(0);
cin >> n;
for (int i = 1; i <= n; i++)
cin >> num[i];
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 = busca_pref(), suf = busca_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 = busca_pref(), suf = busca_suf();
for (int i = add; 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";
}
}
