#include<bits/stdc++.h>
#include<unordered_map>
#define F first
#define S second
using namespace std;
typedef long long ll;
typedef long double D;
const ll inf=(1ll<<61);
const ll mod=1e9+7;
const int MX=1309;
int n,sum;
string s,ret,s1,s2;
int main(){
cin>>n>>s;
if(n%2==0){
puts("NOT POSSIBLE");
return 0;
}
for(int i=0;i<n/2;i++)s1+=s[i];
for(int i=n/2+1;i<n;i++)s2+=s[i];
if(s1==s2){
cout<<s1<<endl;
return 0;
}
int f1=0,f2=0,j=0;
for(int i=0;i<n;i++){
if(s[i]==s1[j])j++;
if(j==n/2){
f1++;
j=0;
}
}
j=0;
for(int i=0;i<n;i++){
if(s[i]==s2[j])j++;
if(j==n/2){
f2++;
j=0;
}
}
if(f1==2&&f2==2){
puts("NOT UNIQUE");
return 0;
}
if(f1==2){
cout<<s1<<endl;
return 0;
}
if(f2==2){
cout<<s2<<endl;
return 0;
}
puts("NOT POSSIBLE");
}
