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 Two_Gram {

	public static void main(String[] args) {
		Scanner SC = new Scanner(System.in);
		int n = SC.nextInt();
		String S = SC.next();
		boolean TwoG = false;
		int C;
		for (int i =0;i<n-1;i++) {
			C = Math.abs(S.charAt(i)-S.charAt(i+1));
			if (C <= 1 || C == 25) {
				for (int j = i+1;j<n-1;j++) {
				  if (S.charAt(j)==S.charAt(i) && S.charAt(j+1) == S.charAt(i+1)) {System.out.println(""+S.charAt(i)+S.charAt(i+1));TwoG=true;break;}}
				if (TwoG) break;}
		}
		if (!TwoG) System.out.println(""+S.charAt(0)+S.charAt(1));
		

	}}
