import sys

is_first_input = False

numbers = []

for line in sys.stdin:
	if not is_first_input:	
		start_input = int(line.rstrip())
		#print ("start_input = ", start_input)
		is_first_input = True
		numbers.clear()
	else:
			#print ("start_input = ", start_input)
		numbers.append(list(map(int, line.rstrip().split(' '))))
		if len(numbers) == start_input: 
			is_first_input = False
			

		# start_input =- N
		# numbers > towns

		win_pairs = 0
		i = 0
		delta = [0,0]
		
		if is_first_input == False:
			
			while len(numbers) != 1:			
				#print("NEW ITERATION")
				#print(numbers)
				for i in range(len(numbers)):
					#print("i = ", i, ", len = ", len(numbers))
					delta[0] = numbers[0][0] - numbers[i][0]
					delta[1] = numbers[0][1] - numbers[i][1]

					if (abs(delta[0])==abs(delta[1]) and delta[0] != 0):
						win_pairs += 2
						#print("win_pairs = ", win_pairs, ", delta[0] = ", delta[0], ", delta[1] = ", delta[1])
		
				numbers.pop(0) 
			#print("winpairs = ", win_pairs, ", all = ", start_input**2)
			output = win_pairs / start_input**2

			print("%.6f" %(output))