import sys
from math import sqrt, pi, sin
inp = lambda : sys.stdin.readline()[:-1]
mii = lambda x = " ": [*map(int,inp().split(x))]
mfi = lambda x = " ": [*map(float,inp().split(x))]
ii = lambda : int(inp())
s = input()
d = []
for i in range(len(s)):
for j in range(i + 1, len(s)+1):
d.append(s[i:j])
d.sort()
print(d[-1])