xa,ya=map(int,input().split())
xb,yb=map(int,input().split())


def f(yp):
    AP=xa**2+(yp-ya)**2
    BP=xb**2+(yp-yb)**2
    return AP+BP

low,high=0,1000

while(high-low>10**(-8)):
    l2=(low*2+high)/3
    h2=(low+high*2)/3
    if f(l2)>f(h2):
        low=l2
    else:
        high=h2
print(low)