#include <bits/stdc++.h>
#define int long long
#define ff first
#define ss second
#define pb push_back
#define all(x) x.begin(),x.end()
using namespace std;
const int N=2e6+5,t=33,mod=1e9+7;
int pw[N],p[N];
char s[N];
bool ok(int l1,int r1,int l2,int r2){
if(l1>r1)return 1;
int h1=(p[r1]-(p[l1-1]*pw[r1-l1+1]));
if(h1<0)h1+=(abs(h1)/mod+1)*mod;
h1%=mod;
int h2=(p[r2]-(p[l2-1]*pw[r2-l2+1]));
if(h2<0)h2+=(abs(h2)/mod+1)*mod;
h2%=mod;
if(h1==h2)return 1;
return 0;
}
signed main(){
ios_base::sync_with_stdio();
cin.tie(0);cout.tie(0);
int n;
cin>>n;
for(int i=1;i<=n;i++){
cin>>s[i];
}
if(n%2==0){
cout<<"NOT POSSIBLE\n";
return 0;
}
pw[0]=1;
for(int i=1;i<=n;i++){
pw[i]=(pw[i-1]*t)%mod;
p[i]=(p[i-1]*t+s[i])%mod;
}
int p1=0,p2=0;
for(int i=1;i<=n;i++){
bool ok1=1,ok2=1;
if(i<=n/2+1){
if(i!=1)ok1=ok(1,i-1,n/2+2,n/2+i);
if(i!=n/2+1)ok2=ok(i+1,n/2+1,n/2+i+1,n);
if(ok1 && ok2 && p1==0)p1=1;
}
else{
ok1=ok(1,1+(i-1-(n/2+1)),n/2+1,i-1);
if(i!=n)ok2=ok(n/2-n+i+1,n/2,i+1,n);
if(ok1 && ok2 && p2==0)p2=1;
}
}
if(p1==0 && p2==0)cout<<"NOT POSSIBLE\n";
else if(p2==0 or ok(1,n/2,n/2+2,n)){
for(int i=n/2+2;i<=n;i++)cout<<s[i];
cout<<"\n";
}
else if(p1==0){
for(int i=1;i<=n/2;i++)cout<<s[i];
cout<<"\n";
}
else cout<<"NOT UNIQUE\n";
}
/*
*/
