# 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_a == x_b:
        break
    elif x_ab <= 2:
        i += 0.5
        j += 0.5
        x_ab -= 1
        break
    else:
        i += 1.0
        j += 1.0
        x_ab -= 2

k = 0
if x_ab > 0:
    k = x_ab

while True:
    if y_a == y_b:
        break
    if y_ab <= 2:
        i += 0.5
        j += 0.5
        y_ab -= 1
        break
    else:
        i += 1.0
        j += 1.0
        y_ab -= 2

if y_ab > 0:
    l = y_ab

print(i + j + k + l)