#coding: utf-8
#yuki_306
import sys

s1=map(int,raw_input().split())
s2=map(int,raw_input().split())

xa=s1[0]
ya=s1[1]
xb=s2[0]
yb=s2[1]

if xa==xb:
 res=1.0*(yb+ya)/2
 print "%.7f" % res
elif ya==yb:
 print ya
 sys.exit()
else:
 xa=-xa
 res=(1.0*ya-1.0*(yb-ya)/(xb-xa)*xa)
 print "%.7f" % res