#include <bits/stdc++.h>
using namespace std;
#define int long long
#define show(x,y) cout << y << " " << #x << endl;
#define show2(x,y,i,j) cout << y << " " << #x << " " << j << " " << #i << endl;
#define show3(x,y,i,j,p,q) cout << y << " " << #x << " " << j << " " << #i << " " << q << " " << #p << endl;
#define show4(x,y) for(auto it:y) cout << it << " "; cout << #x << endl;
typedef pair<int,int>pii;
mt19937 rng(chrono::system_clock::now().time_since_epoch().count());
typedef pair<int,char>pi2;
void solve(){
int n;
cin >> n;
string s;
cin >> s;
if(n%2==0){
cout << "NOT POSSIBLE\n";
return;
}
//front
string cur="";
for(int x=0;x<n/2;x++){
cur.push_back(s[x]);
}
//show(cur,cur);
int ptr=0;
for(int x=n/2;x<n;x++){
if(ptr<n/2&&s[x]==cur[ptr]) ptr++;
}
int check=0;
if(ptr==n/2){
check=1;
}
string cur2="";
for(int x=n/2+1;x<n;x++){
cur2.push_back(s[x]);
}
ptr=0;
for(int x=0;x<n/2+1;x++){
if(ptr<n/2&&s[x]==cur2[ptr]) ptr++;
}
if(ptr==n/2){
check+=2;
}
if(check==1){
cout << cur;
}
else if(check==2){
cout << cur2;
}
else if(check==3&&cur==cur2){
cout << cur;
}
else if(check==3){
cout << "NOT UNIQUE\n";
}
else cout << "NOT POSSIBLE\n";
}
int32_t main(){
ios::sync_with_stdio(0);
cin.tie(0);
int t=1;
//cin >> t;
while(t--){
solve();
}
}
