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

#include <iostream>
#include <algorithm >
#include <math.h>
#include <vector>
#include <string> 
#include <set>
using namespace std;
int b[26] = { 0 };
int a[1000001];
int main() {


	int n, m=0,c,x;
	string s;
	cin >> n >> s;
	for (int i = 0; i < n- 1; i++) {
		c = 0;
		x = 0;
		for (int j = i + 1; j < n - 1; j ++) {
			if (s[i] == s[j] && s[i + 1] == s[j + 1])c++;
		}
		if (c > m) {
			m = c;
			x = i;
		}
	}
	cout << s[x] << s[x + 1];
	return 0;
}

