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.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.HashMap;
import java.util.Map;

public class Compt {

	static BufferedReader br;
	static int max = 0 ;
	static String key = "";
	public static void main(String[] args) throws NumberFormatException, IOException {
		br = new BufferedReader(new InputStreamReader(System.in));
		int n = Integer.parseInt(br.readLine());
		String s = br.readLine();
		Map<String , Integer> map = new HashMap<String, Integer>();
		for (int i = 0 ; i < n - 1 ; i++) {
			String sub = s.substring(i, i+2);
			if(map.containsKey(sub)) {
				int c = map.get(sub) + 1;
				map.remove(sub);
				map.put(sub, c);
			} else {
				map.put(sub, 0);
			}	
		}
		
	    map.forEach((k,v)-> {
	    	if ( v > max ) {
	    		max = v;
	    		key = k;
	    	}
	    	
	    });
		System.out.println(key);
            
	}
}
