def get_line(x, y, nw):
    if nw:
        return x-y
    else:
        return x+y

try:
    while 1:
        N = int(input())
        p = [list(map(int, input().split())) for _ in range(N)]

        mem_nw = {}
        mem_ne = {}
        suma = 0
        for x, y in p:
            nw = get_line(x, y, True)
            ne = get_line(x, y, False)
            if ne in mem_ne:
                mem_ne[ne] += 1
            else:
                mem_ne[ne] = 1

            if nw in mem_nw:
                mem_nw[nw] += 1
            else:
                mem_nw[nw] = 1
        for k in mem_ne:
            p = mem_ne[k]
            suma += p*(p-1)
        for k in mem_nw:
            p = mem_nw[k]
            suma += p*(p-1)
        print(suma/N**2)

except EOFError:
    pass