#!/bin/python

from collections import Counter

def solve():
    num = int(input())
    array = []
    for _ in range(num):
        array.append(list(map(int, input().split())))
    good = 0
    alll = num * num

    c = Counter()  
    c2 = Counter()  

    for item in array:
        c[item[0] - item[1]] += 1
        c2[item[0] + item[1]] += 1

    for i in c:
        good += (c[i] * (c[i]-1))

    for i in c2:
        good += (c2[i] * (c2[i]-1))


    if good == 0:
        print(0)
        return
    print(good/ alll)


def main():
    try:
        while True:
            solve()
    except:
        pass

main()