from math import sqrt
inputs = lambda:map(int, raw_input().split())
xa, ya = inputs()
xb, yb = inputs()

if ya > yb:
    xa, ya, xb, yb = xb, yb, xa, ya
xa2 = xa**2*10**16
xb2 = xb**2*10**16
ya *= 10**8
yb *= 10**8

def calc(yp):
    return sqrt(xa2+(ya-yp)**2) + sqrt(xb2+(yb-yp)**2)

left = ya; right = yb
while 1:
    lm = (2*left+right)/3.
    rm = (left+2*right)/3.
    lc = calc(lm)
    rc = calc(rm)
    if lc < rc:
        if rm==right: break
        right = rm
    else:
        if lm==left: break
        left = lm
print left/10.**8