px,py = map(int,input().split())
qx,qy = map(int,input().split())
ans = 0

if ((px < 0) and (qx > 0)) or ((px > 0) and (qx < 0)):
    ans += (abs(px) + abs(qx))
elif ((px < 0) and (qx < 0)) or ((px > 0) and (qx > 0)):
    ans += (abs(px - qx))
else:
    ans += (abs(px - qx))
    
if ((py < 0) and (qy > 0)) or ((py > 0) and (qy < 0)):
    ans += (abs(py) + abs(qy))
elif ((py < 0) and (qy < 0)) or ((py > 0) and (qy > 0)):
    ans += (abs(py - qy))
else:
    ans += (abs(py - qy))
print(ans / 2)