from decimal import Decimal as d
xa,ya=map(d,input().split())
xb,yb=map(d,input().split())


def f(yp):
    s=d('2')
    AP=(xa**s+(yp-ya)**s).sqrt()
    BP=(xb**s+(yp-yb)**s).sqrt()
    return (AP+BP)

low,high=d('-1000'),d('2000')

D=d('10')**d('-9')

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