shit = raw_input()
lis = shit.split()
a = int(lis[0])
b = int(lis[1])
c = int(lis[2])
d = int(lis[3])
s1 = a*b + c*d
s2 = a*c + b*d
s3 = a*d + b*c

if s1>s2 and s1>s3:
    print(s1)
if s2>s1 and s2>s3:
    print(s2)
if s3>s1 and s3>s2:
    print(s3)