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

n = int(input())
s = input()
maxCount = []
maxTwoGram = []
for i in range(len(s) - 1):
    count = 0
    twoGram = ''
    for j in range(len(s) - 1):
        if s[j] == s[i] and s[j + 1] == s[i + 1]:
            count += 1
    maxCount.append(count)
    maxTwoGram.append(s[i] + s[i + 1])
max_count = maxCount[0]
l = [maxTwoGram[0]]
for i in range(1, len(maxCount)):
    if max_count < maxCount[i]:
        max_count = maxCount[i]
        l[0] = maxTwoGram[i]
print(l[0])
