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.*;
public class Main {
	public static void main(String[]args) {
		Scanner cin=new Scanner(System.in);
		int n;
		String s;
		int num=0;
		n=Integer.parseInt(cin.nextLine());
		s=cin.nextLine();
		System.out.println(s);
		HashMap<String,Integer> hs=new HashMap<String,Integer>();
		while(num<n-1) {
			String temp=""+s.charAt(num)+s.charAt(num+1);
			if(hs.containsKey(temp)) {
				hs.put(temp, hs.get(temp)+1);
			}
			else {
				hs.put(temp, 1);
			}
			num++;
		}
		int result=0;
		for(int value:hs.values()) {
			if(value>result)
				result=value;
		}
		for(String key:hs.keySet()) {
			if(hs.get(key)==result) {
				System.out.println(key);
				break;
			}
		}
	}
}
