#include <iostream>
using namespace std;
int main()
{
ios::sync_with_stdio(0);
cin.tie(0);
int n; string s;
cin >> n >> s;
if(n % 2 == 0)
cout << "NOT POSSIBLE\n";
else
{
string ans;
string t1 = s.substr(1, n/2);
string t2 = s.substr(1+n/2, n/2);
int cnt = 0;
for(int i = 0; i < n/2; i++)
if(t1[i] != t2[i])
cnt++;
if(cnt == 0)
ans = t1;
for(int i = 0; i < n/2; i++)
{
int pre_cnt = cnt;
if(t1[i] != t2[i])
cnt--;
t1[i] = s[i];
if(t1[i] != t2[i])
cnt++;
if(cnt == 0 && pre_cnt > 0)
{
if(ans.empty())
ans = t1;
else
{
cout << "NOT UNIQUE\n";
return 0;
}
}
}
for(int i = n/2; i < n; i++)
{
int pre_cnt = cnt;
if(t1[i-n/2] != t2[i-n/2])
cnt--;
t2[i-n/2] = s[i];
if(t1[i-n/2] != t2[i-n/2])
cnt++;
if(cnt == 0 && pre_cnt > 0)
{
if(ans.empty() || ans == t1)
ans = t1;
else
{
cout << "NOT UNIQUE\n";
return 0;
}
}
}
if(!ans.empty())
cout << ans << '\n';
else
cout << "NOT POSSIBLE\n";
}
}
