n=int(input())
a=0
for i in range(0,n):
	v=input().split()
	x=int(v[0])
	y=int(v[1])
	d=((200-x)**2+(200-y)**2)**0.5

	if d<=200:
		a=a+1
b=4*(a/n)
round(b,4)
b=str(b)
while len(b)<6:
	b=b+"0"
print(b)