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

		win_pairs = 0
		i = 0
		delta = [0,0]
		
		if is_first_input == False:
			
			while len(numbers) != 0:
				tester = numbers[0]
				numbers.pop(0) 			
				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]
					delta[0] = tester[0] - numbers[i][0]
					delta[1] = tester[1] - numbers[i][1]

					if (abs(delta[0])==abs(delta[1]) and delta[0] != 0):
						win_pairs += 2
		
				#numbers.pop(0) 

			output = win_pairs / start_input**2

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