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(raw_input())

twogramstr = map(str, raw_input())[:n]

two_grams = []
two_grams_dict = {}
for i in range(len(twogramstr) - 1):
	twogram = str(twogramstr[i] + twogramstr[i+1])
	two_grams.append(twogram)


for gram in two_grams:
	if gram not in two_grams_dict:
		two_grams_dict[gram] = 1
	else:
		two_grams_dict[gram] += 1

maximum = 0
for key, value in two_grams_dict.iteritems():
	if value > maximum:
		maximum = value

print two_grams_dict.keys()[two_grams_dict.values().index(maximum)]
