#include<bits/stdc++.h>
#define int long long
using namespace std;
int sum[2000005];
int pw[2000005];
int md=1e9+7;
int32_t main(){
ios_base::sync_with_stdio(false);
cin.tie(0);
int n;cin>>n;
string s;cin>>s;
if(n%2==0){
cout<<"NOT POSSIBLE";
return 0;
}
pw[0]=1;
for(int i=1;i<=n;i++){
int x=s[i-1]-'A';
sum[i]=(sum[i-1]*26+x)%md;
pw[i]=(pw[i-1]*26)%md;
//cerr<<sum[i]<<" ";
}
//cerr<<"\n";
map<int,int>mp;
int cnt=0,val=0;
for(int i=1;i<=n;i++){
int l=0,r=0;
int m=n/2+1;
//cerr<<"i:"<<i<<"\n";
if(i<=m){
l=(sum[i-1]*pw[m-i]+(sum[m]-sum[i]*pw[m-i])%md+md)%md;
r=((sum[n]-sum[m]*pw[n-m])%md+md)%md;
//cerr<<"r:"<<r<<"\n";
if(l==r)cnt++,val=i;
//cerr<<l<<" "<<r<<"\n";
}else{
l=sum[m-1];
r=((sum[i-1]-sum[m-1]*(pw[i-m]))%md+md)%md;
//cerr<<"r:"<<r<<"\n";
r=(r*(pw[n-i]))%md;
r+=(sum[n]-sum[i]*(pw[n-i]))%md+md;
r%=md;
if(l==r)cnt++,val=i;
//cerr<<l<<" "<<r<<"\n";
}
if(l==r)mp[r]++;
}
if(cnt==0){
cout<<"NOT POSSIBLE";
}
else if(mp.size()>1){
cout<<"NOT UNIQUE\n";
}else{
int cur=0;
string ans;
int cnt=0;
for(int i=0;i<n;i++){
if(i+1==val)continue;
cnt++;
ans.push_back(s[i]);
if(cnt==n/2)break;
}
cout<<ans<<"\n";
}
}
