#include <bits/stdc++.h>
using namespace std;
#define fio() ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL)
int n,m,b[200005],a[200005],k,flag,t,mpp[50],bir,iki,uc,cev;
char c;
string s,ss,yaz,s1,s2;
vector<int> v;
int main(void){
fio();
cin>>n>>s;
if(n%2==0){cout<<"NOT POSSIBLE\n";return 0;}
for(int i=0;i<n;i++){
mpp[s[i]-'A']++;
}
for(int i=0;i<=25;i++){
if(mpp[i]%2){
cev++;
c=i+'A';
}
}
if(cev!=1){cout<<"NOT POSSIBLE\n";return 0;}
for(int i=0;i<n;i++){
if(s[i]==c)v.push_back(i);
}
cev=0;
for(int i=0;i<n/2;i++){
s1+=s[i];
}
for(int i=n/2;i<n;i++)s2+=s[i];
string last=s1;
int son=(int)v.size()-1;
while(v[son]>=n/2){
if(flag==0){
string s3=s2;
s3.erase(s3.begin()+v[son]-n/2);
if(s3==s1){
cev++;
flag=1;
}
}
son--;
}
s1.clear();
s2.clear();
for(int i=0;i<=n/2;i++){
s1+=s[i];
}
for(int i=n/2+1;i<n;i++){
s2+=s[i];
}
if(s2!=last){
while(son>=0){
string s3=s1;
s3.erase(s3.begin()+v[son]);
//~ cout<<s3<<endl;
if(s3==s2){
cev++;
break;
}
son--;
}
}
if(cev==2){
cout<<"NOT UNIQUE\n";
return 0;
}
if(cev==0){
cout<<"NOT POSSIBLE\n";
return 0;
}
if(flag==1){
for(int i=0;i<n/2;i++)cout<<s[i];
return 0;
}
for(int i=n/2+1;i<n;i++)cout<<s[i];
return 0;
}
