#include<bits/stdc++.h>
using namespace std;
const int nmax=2e6+42,base=29,mod=1e9+7;
string inp;
string outp;
int n,sz;
int st[nmax],pref[nmax];
set<int> seen;
int ask(int l,int r)
{
if(l>r)return 0;
long long help=pref[r];
long long sub=0;
if(l)sub=pref[l-1];
sub=sub*st[r-l+1]%mod;
//cout<<"ask "<<l<<" "<<r<<" -> "<<((help-sub)%mod+mod)%mod<<endl;
return ((help-sub)%mod+mod)%mod;
}
void pick(int l,int r)
{
int now=ask(l,r);
if(seen.count(now))return;
seen.insert(now);
if(seen.size()>=2)
{
cout<<"NOT UNIQUE"<<endl;
exit(0);
}
for(int i=l;i<=r;i++)
outp.push_back(inp[i]);
}
int main()
{
ios_base::sync_with_stdio(false);
cin.tie();
cout.tie();
cin>>sz;
cin>>inp;
n=(sz-1)/2;
pref[0]=inp[0]-'A';
for(int i=1;i<sz;i++)
pref[i]=(1LL*pref[i-1]*base+inp[i]-'A')%mod;
st[0]=1;
for(int i=1;i<sz;i++)st[i]=1LL*st[i-1]*base%mod;
//in the first half
for(int i=0;i<=n;i++)
{
//cut i-th
long long part1=ask(0,i-1)*st[n-i];
long long part2=ask(i+1,n);
part1=(part1+part2)%mod;
if(part1==ask(n+1,sz-1))
pick(n+1,sz-1);
}
//in the second half
for(int i=n;i<sz;i++)
{
//cut i-th
long long part1=ask(n,i-1)*st[sz-1-i];
long long part2=ask(i+1,sz-1);
part1=(part1+part2)%mod;
if(part1==ask(0,n-1))
pick(0,n-1);
}
if(outp.size())cout<<outp<<endl;
else cout<<"NOT POSSIBLE"<<endl;
return 0;
}
