import math


def solve():

    p = input().split(' ')
    px = int(p[0])
    py = int(p[1])

    q = input().split(' ')
    qx = int(q[0])
    qy = int(q[1])

    t = getDistance(px, py, qx, qy)

    dx = t[0] / 2
    dy = t[1] / 2

    if (t[0] % 2 != 0 and t[1] % 2 != 0):
        dx = math.ceil(dx)
        dy = math.floor(dy)

    print(dx + dy)

def getDistance(x1, y1, x2, y2):
    return (abs(x2 - x1), abs(y2 - y1))


solve()