p1,p2 = map(int,input().split())
q1,q2 = map(int,input().split())
time,timex,timey = 0,0,0
if(p1>q1):
    timex = (p1-q1)/2
elif(p1<q1):
    timex = (q1-p1)/2
elif(p1==q1):
    timex = 0
if(p2>q2):
    timey = (p2-q2)/2
elif(p2<q2):
    timey = (q2-p2)/2
elif(p2==q2):
    timey = 0
if(timex<0):
    timex *= -1
if(timey<0):
    timey *= -1
time = timex + timey
print(time)