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

def prefix_function(s):
	n = len(s)
	pi = [0 for i in range(n)]
	for i in range(1, n):
		j = pi[i - 1]
		while (j > 0 and s[i] != s[j]):
			j = pi[j - 1]
		if (s[i] == s[j]):
		  j += 1
		pi[i] = j
	summ = 0
	for i in pi:
		if (i == 2):
			summ += 1
	return summ

s = set()
maxx = 0
n = int(input())
st = input()
it = ""
for i in range(1, n):
	d = st[i - 1] + st[i]
	if (d not in s):
		s.add(d)
		q = prefix_function(d + "#" + st)
		if (q > maxx):
			it = d
			maxx = q

print(it)