#!/usr/bin/python

try:
    while True:
        N = int(input())
        towns = [tuple(map(int, input().split(' '))) for _ in range(N)]

        s = dict()
        ss = dict()
        for t in towns:
            r = t[0] - t[1]
            if r in s:
                s[r].append(t)
            else:
                s[r] = [t]

            r = t[0] + t[1]
            if r in ss:
                ss[r].append(t)
            else:
                ss[r] = [t]

        win = 0
        for v in s.values():
            win += len(v) * (len(v) - 1)
        for v in ss.values():
            win += len(v) * (len(v) - 1)
        print(win / (N*N))

except EOFError:
    pass