#include <algorithm>
#include <iostream>
#include <iomanip>
#include <numeric>
#include <cassert>
#include <bitset>
#include <vector>
#include <cmath>
#include <queue>
#include <stack>
#include <set>
#include <map>
#define syosu(x) fixed<<setprecision(x)
using namespace std;
typedef long long ll;
typedef unsigned int uint;
typedef unsigned long long ull;
typedef pair<int,int> P;
typedef pair<double,double> pdd;
typedef pair<ll,ll> pll;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<double> vd;
typedef vector<vd> vvd;
typedef vector<ll> vl;
typedef vector<vl> vvl;
typedef vector<string> vs;
typedef vector<P> vp;
typedef vector<vp> vvp;
typedef vector<pll> vpll;
typedef pair<P,int> pip;
typedef vector<pip> vip;
const int inf=1<<30;
const ll INF=1ll<<60;
const double pi=acos(-1);
const double eps=1e-8;
const ll mod=1e9+7;
const int dx[4]={-1,0,1,0},dy[4]={0,-1,0,1};
string f(string s,string t){
int n=t.size();
int L=0,R=n;
while(L<n&&s[L]==t[L]) L++;
while(R>0&&s[R]==t[R-1]) R--;
if(L<R) return "";
return t;
}
int n;
string s;
int main(){
cin>>n>>s;
if(n%2==0){
cout<<"NOT POSSIBLE"<<endl;
return 0;
}
auto t1=f(s.substr(n/2,n/2+1),s.substr(0,n/2));
auto t2=f(s.substr(0,n/2+1),s.substr(n/2+1,n/2));
if(t1==""&&t2=="") cout<<"NOT POSSIBLE"<<endl;
else if(t1!=""&&t2!=""&&t1!=t2) cout<<"NOT UNIQUE"<<endl;
else cout<<(t1==""?t2:t1)<<endl;
}
