"""
２点間の距離は
abs(Px-Qx) + abs(Py-Qy)

最短時間は
(abs(Px-Qx) + abs(Py-Qy))/2

"""

Px,Py = (int(i) for i in (input().split()))

Qx,Qy = (int(i) for i in (input().split()))

print((abs(Px-Qx) + abs(Py-Qy))/2)