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

import java.util.HashSet;
import java.io.PrintWriter;
import java.util.*;
public class Final {
		public static void main(String[] args) {
		Scanner sc =new Scanner(System.in);
		PrintWriter out=new PrintWriter(System.out);
		int n=sc.nextInt();
		String s=sc.next();
		String ss=s.substring(0,2);int max=0;
		for(int i=0;i<n-2;i++) {
			String st=s.substring(i,i+2);int co=0;
			for(int j=0;j<n-2;j++) {
				String sr=s.substring(j,j+2);
				if(st.equals(sr))
					co++;
			}
			if(max<co) {
				max=co;
				ss=st;
			}
		}
		out.println(ss);
		out.flush();
	}

}
