import io
import sys
from collections import defaultdict, deque, Counter
from itertools import permutations, combinations, accumulate
from heapq import heappush, heappop
from bisect import bisect_right, bisect_left
from math import gcd
import math

_INPUT = """\
6
3
1 4
1 5
9 2
2
1 1
2 2
5
4 4
5 7
0 4
2 10
-6 4
"""

class BIT:
    def __init__(self, n):
        self._n = n
        self.data = [0] * n
    def add(self, p, x):
        assert 0 <= p < self._n
        p += 1
        while p <= self._n:
            self.data[p - 1] += x
            p += p & -p
    #合計にはrを含まない
    def sum(self, l, r):
        assert 0 <= l <= r <= self._n
        return self._sum(r) - self._sum(l)
    def _sum(self, r):
        s = 0
        while r > 0:
            s += self.data[r - 1]
            r -= r & -r
        return s
    #pの位置をxという値にセット
    def set(self, p, x):
        self.add(p, -self.sum(p, p+1) + x)

def input():
  return sys.stdin.readline()[:-1]

def solve(test):
  N=int(input())
  data=[list(map(int, input().split())) for _ in range(N)]
  d={x:i for i,x in enumerate(sorted(set([x for x,y in data]+[y for x,y in data])))}
  data=[(d[x],d[y]) for x,y in data]
  data.sort(key=lambda x:(x[0],x[1]))
  bit=BIT(len(d))
  Q=0
  for x,y in data:
    Q+=bit.sum(y+1,len(d))
    bit.add(y,1)
  bit=BIT(len(d))
  data.sort(key=lambda x:(x[0],-x[1]))
  # print(data)
  P=0
  for x,y in data:
    P+=bit.sum(0,y)
    bit.add(y,1)
  C1=Counter([x for x,y in data])
  C2=Counter([y for x,y in data])
  R=sum([(N-C1[x])*C1[x] for x in C1])//2
  S=sum([(N-C2[y])*C2[y] for y in C2])//2
  print((P-Q)/(R*S)**0.5)

def random_input():
  from random import randint,shuffle
  N=randint(1,10)
  M=randint(1,N)
  A=list(range(1,M+1))+[randint(1,M) for _ in range(N-M)]
  shuffle(A)
  return (" ".join(map(str, [N,M]))+"\n"+" ".join(map(str, A))+"\n")*3

def simple_solve():
  return []

def main(test):
  if test==0:
    solve(0)
  elif test==1:
    sys.stdin = io.StringIO(_INPUT)
    case_no=int(input())
    for _ in range(case_no):
      solve(0)
  else:
    for i in range(1000):
      sys.stdin = io.StringIO(random_input())
      x=solve(1)
      y=simple_solve()
      if x!=y:
        print(i,x,y)
        print(*[line for line in sys.stdin],sep='')
        break

#0:提出用、1:与えられたテスト用、2:ストレステスト用
main(0)