x, y = map(int, input().split())
xx, yy = map(int, input().split())
from math import sqrt


def f(c):
    # (0,c)
    return sqrt((y - c) ** 2 + x**2) + sqrt((yy - c) ** 2 + xx**2)


l = 0
r = 10**9

while r - l > 10**-6:
    c1 = (l * 2 + r) / 3
    c2 = (l + r * 2) / 3
    if f(c1) <= f(c2):
        r = c2
    else:
        l = c1
print(r)