# coding=utf-8:

a = list(map(int, input().split()))
b = list(map(int, input().split()))

x_a = a[0]
x_b = b[0]
y_a = a[1]
y_b = b[1]

x_ab = abs(x_a - x_b) / 2 
y_ab = abs(y_a - y_b) / 2

i = 0.0
j  = 0.0
k = 0.0
l = 0.0
while True:
    if x_ab <= 2:
        break
    else:
        i += 1.0
        j += 1.0
        x_ab -= 2


while True:
    if y_ab <= 2:
        break
    else:
        i += 1.0
        j += 1.0
        y_ab -= 2

k = x_ab
l = y_ab

print(i + j + k + l)