import math
def calc(x,y):
	return math.sqrt(x * x + y * y)
xa,ya = map(int,raw_input().split())
xb,yb = map(int,raw_input().split())
if ya > yb:
	ya,yb,xa,xb = yb,ya,xb,xa
high = yb * 1.0
low = ya * 1.0
mid = (high + low) / 2.0
pre = calc(mid - ya,xa) + calc(yb - mid,xb)
for i in range(1 << 10):
	mid = (high + low) / 2.0
	#print mid
	res = calc(mid - ya,xa) + calc(yb - mid,xb)
	if res <= pre:
		high = mid
		pre = res
	else:
		low = mid
print mid