def gyoretu(n1, n2, n3, n4):
    return n1*n4-n2*n3

s1 = input().rstrip().split(' ')
s2 = input().rstrip().split(' ')
n1 = int(s1[0])
n2 = int(s1[1])
n3 = int(s2[0])
n4 = int(s2[1])

print(gyoretu(n1, n2, n3, n4))