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;

public class Twogram_CodeForces {
	
	public static void main(String[] args) throws NumberFormatException, IOException {
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		int n = Integer.parseInt(br.readLine());
		String s =br.readLine();
		String [] subs = new String [n*2];
		int [] freq= new int [n*2];
		int j=0;
		int  min=n;
		String minString=s;
		for(int i=0;i<n-1;i++){
			String temp=s;
			String substring = s.substring(i,i+2);
			if(i<n-2&&s.charAt(i+2)==s.charAt(i)&&s.charAt(i)==s.charAt(i+1))
				temp=temp.replace(s.substring(i,i+3), "");
			else 
				temp=temp.replaceAll(substring, "");
			if(temp.length()<min){
				min=temp.length();
				minString=substring;
			}
			subs[j]=substring;
			j++;
		}
		System.out.println(minString);
		String g=  "GGRRAATTZZZ";
		String t = g.replaceAll("ZZ","");
		////System.out.println(t);
	}
	

}
