#include <iostream>
#include <vector>
#include <map>
#include <set>
#include <math.h>
#include <string>
#include <algorithm>
#include <random>
#include <iomanip>
#include <utility>
#include <cstring>
//#include <bits/stdc++.h>
#define ll long long
#define s .size()
using namespace std;
const ll N=1e5+9,M=1e18+9,mod=1e9+7;
//cout<<fixed<<setprecision(6)<<
ll n,x,y;
string a,b;
map<char,ll>m;
int main()
{
ios::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
cin>>n>>a;
if(!(n%2))goto end;
for(ll i=0;i<n;++i)++m[a[i]];
for(auto i:m)if(i.second%2)++x,y=i.second;
end:
if(x!=1)cout<<"NOT POSSIBLE";
else if(y>1)cout<<"NOT UNIQUE";
else{
for(ll i=0;i<n;++i)
if(m[a[i]]>1)b+=a[i];
n/=2;
for(ll i=0;i<n;++i)cout<<b[i];
}
}
