Px , Py = map(int, input().split())
Qx , Qy = map(int, input().split())
sum = 0
if Px * Qx < 0:
    sum += (abs(Px) + abs(Qx)) / 2
else:
    sum += abs((Px - Qx) / 2)
if Py * Qy < 0:
    sum += (abs(Py) + abs(Qy)) / 2
else:
    sum += abs((Py - Qy) / 2)

print(sum)