def num_cons(pointA, coords):
	cons = 0
	for pointB in coords:
		if pointB[1] == pointA[1] and pointB[0] == pointA[0] or (pointB[1] - pointA[1]) < 0:
			pass
		else:
			if (pointB[1] - pointA[1]) == abs(pointB[0] - pointA[0]):
				cons = cons + 1
	return cons

def solve(coords):
	wins = 0
	for i in coords:
		wins = wins + 2*num_cons(i, coords)
		
	result = wins/pow(len(coords),2)
	
	print("%.6f" % result)

loop = True

while loop:
	
	try:
		
		coords = []

		N = int(input(""))
		
		for i in range(N):
			coords.append(input().split(" "))
			coords[i][0] = int(coords[i][0])
			coords[i][1] = int (coords[i][1])


		solve(coords)
	except Exception as e:
		loop = False