import sys
if sys.platform =='ios':
	import clipboard
	a=clipboard.get()
	a = a.split('\n')
	text = '\n'.join(a)
	with open('input_file.txt','w') as f:
		f.write(text)
	sys.stdin = open('input_file.txt')
	
x1,y1 = map(int,input().split())
x2,y2 = map(int,input().split())
a = abs(y2-y1)/(x2+x1)
if y2 == y1:
	print(y1)
	exit()
if y1 < y2:
	y = a*(-x2)+y2
else:
	y = a*(-x1)+y1
print(y)