#include <bits/stdc++.h>
int main()
{
std::ios::sync_with_stdio(false);
std::cin.tie(0);
std::cout.tie(0);
int N;
std::cin>>N;
if(!(N&1)){
std::cout<<"NOT POSSIBLE\n";
return 0;
}
int size = (N-1)/2;
std::string s;
std::cin>>s;
int skips[2]={};
int cur1=0,cur2=N-size;
while(cur2!=N){
if(s[cur1]==s[cur2]){
++cur1;
++cur2;
}else {
++skips[0];
if(skips[0]==2){
break;
}
++cur1;
}
}
if(cur1!=size+1)skips[0]++;
cur1=N-size-1,cur2=0;
while(cur2!=size){
if(s[cur1]==s[cur2]){
++cur1;
++cur2;
}else {
++skips[1];
if(skips[1]==2){
break;
}
++cur1;
}
}
if(cur1!=N)skips[1]++;
//std::cout<<skips[0]<<" "<<skips[1]<<"\n";
if(skips[0]>1&&skips[1]>1){
std::cout<<"NOT POSSIBLE\n";
return 0;
}else if(skips[0]<2&&skips[1]<2){///caso especial: ambas as strings sao iguais
int cur1=0,cur2=N-size;
while(cur2!=N){
if(s[cur1]!=s[cur2]){
std::cout<<"NOT UNIQUE\n";
return 0;
}
++cur1;
++cur2;
}
for(int i=0;i!=size;++i){
std::cout<<s[i];
}
std::cout<<"\n";
}else {
if(skips[1]<2){
for(int i=0;i!=size;++i){
std::cout<<s[i];
}
std::cout<<"\n";
}else {
for(int i=N-size;i!=N;++i){
std::cout<<s[i];
}
std::cout<<"\n";
}
}
}
