15
MIRZOYANOVECLOX
 23
AABBBAAACCCCCAAADDDDDDD
 2
QA
 11
GGRRAATTZZZ
 6
AZAZAZ
 10
SQSQSQSQTG
 3
KEK
 100
URXCAIZFIBNJTPCZHBQIBCILLPXZCFGMKKZMNPLCYGAVJVIBMCZEBSJWPSCPQDYCTTKPOKIJRSKIZPDGCHVOUTMPNECYORSFZFNC
 3
LOL
 2
WW
 8
PUTINVOR
 26
QWERTYUIOPASDFGHJKLZXCVBNM
 5
AZAZA
 9
EGORLETOV
 7
KADUROV
 9
MIKEPIDOR
 9
AAAAAAAAA
 100
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
 5
FUFEL
 50
NYQAHBYYOXLTRYQDMVENEMAQNBAKGLGQOLXNAIFNQTOCLNNQIA

MI
 DD
 QA
 ZZ
 AZ
 SQ
 KE
 IB
 LO
 WW
 PU
 QW
 AZ
 EG
 KA
 MI
 AA
 AA
 FU
 YQ

/*
string sort--> y=string(x.rbegin(),x.rend());
sort array descending --> sort(name,name+size);
sort array ascending --> sort(name,name+size,greater<int>());
replace form string --> x.replace(index,number of move);
*/

#include <iostream>
#include <cmath>
#include <string>
#include <conio.h>
#include <bits/stdc++.h>
using namespace std;
int main()
{
/*
string sort--> y=string(x.rbegin(),x.rend());
-------------------------------------------------
sort array descending --> sort(name,name+size);
------------------------------------------------------------------------------------------------
sort array ascending --> sort(name,name+size,greater<int>());
------------------------------------------------------------------------------------------------
replace form string --> x.replace(index,number of move);
delete the word that being identical --> do 3 for loop the first from 0 to power of size of string and the inner two for loop to delete the words that being identical:
string x;
cin>>x;
int c=0;
while(c<pow(x.size(),2))
	{

		for(int i=0;i<x.size();i++)
			for(int j=0;j<x.size();j++)
				if(i!=j && x[i]==x[j])
					x.erase(j,1);
		c++;
	}
	------------------------------------------------
	long long int n,t,i=1;
cin>>n>>t;
for(i=1;i<=t;i++)
{
if(n%10==0)
	n/=10;
else
	n--;
}
cout<<n<<endl;

*/

long long n,count=0;
cin>>n;
char s[n],c[n];
gets(s);
for(int i=0;i<n;i++)

	{
		for(int j=0;j<n;j++)

			{
				if(i!=j && s[i]==s[j])

					{
						c[count]=s[i];
						count++;
					}
			}

	}
cout<<c[0]<<c[1]<<endl;
return 0;
}