#include <bits/stdc++.h>
using namespace std;
#pragma GCC optimize("O2,no-stack-protector,unroll-loops,fast-math")
//#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
//#pragma GCC optimize("Os")
#define F first
#define S second
#define pb push_back
#define SZ(x) (ll)(x.size())
#define all(x) x.begin(),x.end()
//#define mp make_pair
typedef long long ll;
typedef long double ld;
typedef pair<ll,ll> pll;
//mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
const ll maxn=2e6+10, maxm=2e3+10, lg=22, mod=1e9+9, inf=1e18;
int n,i,j;
string s,t,ans;
map<string,bool> mp;
int main(){
ios_base::sync_with_stdio(0),cin.tie(0),cout.tie(0);
cin>>n;
cin>>s; s='#'+s;
if(n%2==0) return cout<<"NOT POSSIBLE",0;
for(i=1,j=0;i<=n/2;i++) t+=s[i];
for(;i<=n;i++) j+=(s[i]==t[j]);
if(j==n/2) mp[ans=t]=1;
for(t="",i=(n/2)+2;i<=n;i++) t+=s[i];
for(i=1,j=0;i<=n/2+1;i++) j+=(s[i]==t[j]);
if(j==n/2) mp[ans=t]=1;
if(mp.empty()) cout<<"NOT POSSIBLE";
else if(SZ(mp)>1) cout<<"NOT UNIQUE";
else cout<<ans;
return 0;
}
