#include <iostream>
#include <set>
using namespace std;
#define ll long long
const ll N=10000;
ll pre[N],pw[N],base=717,mod=1e9+9;
ll pre1[N],pw1[N],base1=911,mod1=1e9+7;
int get(int l,int r)// one base indexing 1<=l<=n
{
if(r<l)
return 0;
return ((pre[r]-(pre[l-1]*pw[r-l+1])%mod)%mod+mod)%mod;
}
int get1(int l,int r)// one base indexing 1<=l<=n
{
if(r<l)
return 0;
return ((pre1[r]-(pre1[l-1]*pw1[r-l+1])%mod1)%mod1+mod1)%mod1;
}
int main()
{
int n;
cin>>n;
string s;
cin>>s;
pre[0]=0;
pw[0]=1;
for(int i=1;i<=n;i++)
pw[i]=(pw[i-1]*base)%mod;
for(int i=1;i<=n;i++)
pre[i]=((pre[i-1]*base)%mod+(s[i-1]-'A'))%mod;
pre1[0]=0;
pw1[0]=1;
for(int i=1;i<=n;i++)
pw1[i]=(pw1[i-1]*base1)%mod1;
for(int i=1;i<=n;i++)
pre1[i]=((pre1[i-1]*base1)%mod1+(s[i-1]-'A'))%mod1;
set<int> posa,pp;
int len=(n-1)/2;
for(int i=1;(n%2==1) and i<=n;i++)
{
ll fit=-1;
ll sec=-1;
ll fit1=0;
ll sec1=0;
ll hash=0;
int len1=0;
for(int j=1;j<=n;j++)
{
if(j==i)
continue;
hash=(hash*base)%mod;
hash=(hash+s[j-1]-'A')%mod;
len1++;
if(len==len1)
{
len1=0;
if(fit==-1)
fit=hash;
else
sec=hash;
hash=0;
}
}
// int lf=(i-1);
// if(lf<=len)
// {
// fit=((get(1,i-1)*pw[len-lf])%mod+get(i+1,i+len-lf))%mod;
// sec=get(i+len-lf+1,n);
// fit1=((get1(1,i-1)*pw1[len-lf])%mod1+get1(i+1,i+len-lf))%mod1;
// sec1=get1(i+len-lf+1,i+len-lf+len);
// }
// else{
// fit=get(1,len);
// sec=((get(len+1,i-1)*pw[n-i])%mod + get(i+1,n))%mod;
// fit1=get1(1,len);
// sec1=((get1(len+1,i-1)*pw1[n-i])%mod1 + get1(i+1,n))%mod1;
// }
if(fit==sec and fit1==sec1)
{
pp.insert(i);
posa.insert(fit);
}
}
if(posa.size()==0 or n%2!=1)
{
cout<<"NOT POSSIBLE\n";
}
else if(posa.size()==1)
{
int j=*begin(pp);
for(int i=1,p=0;p<len;i++)
{
if(i!=j)
{
cout<<s[i-1];
p++;
}
}
cout<<endl;
}
else{
cout<<"NOT UNIQUE\n";
}
return 0;
}
