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

#include <bits/stdc++.h>
#define F first
#define S second
#define Mp make_pair
#define Pb push_back
#define Tree int h,int l,int r,int L,int R
#define left 2*h,l,(l+r)/2,L,R
#define right 2*h+1,(l+r)/2+1,r,L,R
using namespace std;
int n,f[700],x;
string s;
main ()
{
	cin>>n;
	cin>>s;
	
	for (int i=1; i<s.size(); i++)
	{
		x=(s[i]-'A')*26+(s[i-1]-'A');
		f[x]++;	
		
		//cout<<x<<endl;
	}
	
	int m=701;
	
	for (int i=0; i<=700; i++)
		if (f[i]>f[m]) m=i;
		
//	cout<<f[m]<<endl;	
		
	cout<<char(m/26+'A')<<char(m%26+'A')<<endl;
}