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())
string = input()
t = string[0]
countTemp = 0
count = 0
stringFinal = ""

for i in range(1,n):
	t += string[i];
	t = t[-2] + t[-1]
	temp = string[i-1]
	#if ( t == "ab"):
	#	print ("t :" +t)
	#	print("i :" + str(i))
	for j in range(i,n):
		temp += string[j]
		temp = temp[-2] + temp[-1]
		#if ( i == 1):
			#print ( "temp :" + temp )
			#print ( "i : " + str(i), "j :" + str(j))
		#	print (temp)
		if ( temp == t ):
			countTemp += 1
	if ( countTemp > count ):
		count = countTemp
		stringFinal = t;
	countTemp = 0
print (stringFinal)