#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<ll, ull> plu;
int get_num(char c)
{
return c-'A'+1;
}
struct polyhash
{
ll base=31;
ll mod=1e9+7;
vector<ll> pow1;
vector<ull> pow2;
vector<ll> pref1;
vector<ull> pref2;
void make_hash(string s)
{
ll len=s.length();
pow1.resize(len+1);
pow1[0]=1;
pow2.resize(len+1);
pow2[0]=1;
for(int i=1; i<=len; i++)
{
pow1[i]=(pow1[i-1]*base)%mod;
pow2[i]=pow2[i-1]*base;
}
pref1.resize(len);
pref2.resize(len);
pref1[0]=get_num(s[0]);
pref2[0]=get_num(s[0]);
for(int i=1; i<len; i++)
{
pref1[i]=(pref1[i-1]*base+get_num(s[i]))%mod;
pref2[i]=pref2[i-1]*base+get_num(s[i]);
}
}
plu get_hash(ll L, ll R)
{
ll len=R-L+1;
if(L==0)
{
return {pref1[R], pref2[R]};
}
ll h1=(pref1[R]-(pref1[L-1]*pow1[len])%mod)%mod;
if(h1<0) h1+=mod;
ull h2=pref2[R]-pref2[L-1]*pow2[len];
return {h1, h2};
}
plu reun(ll L1, ll R1, ll L2, ll R2)
{
plu hash1=get_hash(L1, R1);
plu hash2=get_hash(L2, R2);
ll len=R2-L2+1;
ll h1=((hash1.first*pow1[len])%mod+hash2.first)%mod;
if(h1<0) h1+=mod;
ull h2=hash1.second*pow2[len]+hash2.second;
return {h1, h2};
}
};
polyhash h;
string s;
ll n, it, cnt;
plu hash1, hash2, ans;
int main()
{
cin>>n;
cin>>s;
h.make_hash(s);
if(n%2==0)
{
cout<<"NOT POSSIBLE"<<endl;
return 0;
}
hash1=h.get_hash(0, n/2-1);
hash2=h.get_hash(n/2+1, n-1);
if(hash1==hash2)
{
ans=hash1;
it=0;
cnt=1;
}
for(int i=0; i<n/2; i++)
{
if(i==0)
{
hash1=h.get_hash(1, n/2);
}
else hash1=h.reun(0, i-1, i+1, n/2);
if(hash1==hash2)
{
if(hash1!=ans) cnt++;
ans=hash1;
it=1;
}
}
hash1=h.get_hash(0, n/2-1);
for(int i=n/2+1; i<n; i++)
{
if(i==n-1)
{
hash2=h.get_hash(n/2, i-1);
}
else
{
hash2=h.reun(n/2, i-1, i+1, n-1);
}
if(hash1==hash2)
{
if(ans!=hash1) cnt++;
ans=hash1;
it=0;
}
}
if(cnt==0)
{
cout<<"NOT POSSIBLE"<<endl;
return 0;
}
if(cnt>1)
{
cout<<"NOT UNIQUE"<<endl;
return 0;
}
if(it==0)
{
for(int i=0; i<n/2; i++) cout<<s[i];
cout<<endl;
return 0;
}
for(int i=n/2+1; i<n; i++) cout<<s[i];
cout<<endl;
return 0;
}
