a,b=[int(i) for i in input().split()]
c,d=[int(i) for i in input().split()]
if (a<0) and (b<0) and (c<0) and (d<0):
    print((a+b+c+d)*-1/2)
elif (a<0) or (c<0):
    print(((a+c)*-1+(b+d))/2)
elif (b<0) or (d<0):
    print(((b+d)*-1+(a+c))/2)