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

// TLE? (hope not)
// machhar ki maa ki c...
import java.util.*;
import java.util.regex.*;

public class Codechef
{
	public static void main (String[] args)
	{
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		String s = sc.next();
		int count = 0;
		String ans="";
		for(int i = 0; i < s.length()-1; i++)
		{
		    String sub = "" + s.charAt(i) + s.charAt(i+1);
		    Pattern pat = Pattern.compile(sub);
		    Matcher mat = pat.matcher(s);
		    int cin = 0;
		    while(mat.find())
    		{
    		    cin++;
    		}
    		if(count < cin)
    		{
    		    count = cin;
    		    ans = sub;
    		}
		}
		System.out.println(ans);
	}
}
