#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#include <bits/stdc++.h>
#define rc(x) return cout<<x<<endl,0
#define pb push_back
#define mkp make_pair
#define in insert
#define er erase
#define fd find
#define fr first
#define sc second
typedef long long ll;
typedef long double ld;
const ll INF=0x3f3f3f3f3f3f3f3f;
const ll llinf=(1LL<<62);
const int inf=(1<<30);
const int nmax=2e6+50;
const int mod=1e9+7;
using namespace std;
int n,i,j,tot;
string s1,s2,rs;
char c[nmax];
map<string,bool>mp;
void sol(string x,string y)
{
if(x!=y)return;
if(!mp[x])
{
mp[x]=1;
tot++;
rs=x;
}
}
int main()
{
//freopen("sol.in","r",stdin);
//freopen("sol.out","w",stdout);
//mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
ios_base::sync_with_stdio(false);cin.tie(0);cerr.tie(0);cout.tie(0);
cin>>n;
for(i=1;i<=n;i++)cin>>c[i];
if(n%2==0)rc("NOT POSSIBLE");
for(i=1;i<=n/2;i++)s1+=c[i];
for(;i<n;i++)s2+=c[i];
sol(s1,s2);
s1=s2="";
for(i=(n/2)+2;i<=n;i++)s1+=c[i];
for(i=2;i<=(n/2)+1;i++)s2+=c[i];
sol(s1,s2);
s1=s2="";
for(i=1;i<=n/2;i++)s1+=c[i];
j=0;
for(;i<=n;i++)if(c[i]==s1[j])j++;
if(j==n/2)sol(s1,s1);
s1="";
for(i=(n/2)+2;i<=n;i++)s1+=c[i];
j=0;
for(i=1;i<=(n/2)+1;i++)if(c[i]==s1[j])j++;
if(j==n/2)sol(s1,s1);
if(!tot)cout<<"NOT POSSIBLE"<<endl;
else if(tot>1)cout<<"NOT UNIQUE"<<endl;
else cout<<rs<<endl;
return 0;
}
