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

#!/usr/bin/env python3
from sys import stdin
import operator


def solve(tc):
    n = int(stdin.readline().strip())
    s = stdin.readline().strip()

    dic = {}
    for i in range(1,n):
        sub = s[i-1] + s[i]
        if sub in dic:
            dic[sub] += 1
        else:
            dic[sub] = 1
        
    print(max(dic.items(), key=operator.itemgetter(1))[0])


LOCAL_TEST = not __debug__
if LOCAL_TEST:
    infile = __file__.split('.')[0] + "-test.in"
    stdin = open(infile, 'r')

tcs = (int(stdin.readline().strip()) if LOCAL_TEST else 1)
tc = 1
while tc <= tcs:
    solve(tc)
    tc += 1