class SegTree:
    def __init__(self, op, e, n, v=None):
        self._n = n
        self._op = op
        self._e = e
        self._log = (n - 1).bit_length()
        self._size = 1 << self._log
        self._d = [self._e()] * (self._size << 1)
        if v is not None:
            for i in range(self._n):
                self._d[self._size + i] = v[i]
            for i in range(self._size - 1, 0, -1):
                self._d[i] = self._op(self._d[i << 1], self._d[i << 1 | 1])

    def set(self, p, x):
        p += self._size
        self._d[p] = x
        while p:
            l, r = p, p^1
            if l > r: l, r = r, l
            self._d[p >> 1] = self._op(self._d[l], self._d[r])
            p >>= 1

    def get(self, p):
        return self._d[p + self._size]

    #[l, r)の区間で求める
    def prod(self, l, r):
        sml, smr = self._e(), self._e()
        l += self._size
        r += self._size
        while l < r:
            if l & 1:
                sml = self._op(sml, self._d[l])
                l += 1
            if r & 1:
                r -= 1
                smr = self._op(self._d[r], smr)
            l >>= 1
            r >>= 1
        return self._op(sml, smr)

    def all_prod(self):
        return self._d[1]

def op(x, y):
    return x+y

def e():
    return 0

import math
import sys
input = sys.stdin.readline
N = int(input())
XY = [list(map(int, input().split())) for _ in range(N)]
s = set()
for x, y in XY:
    s.add(x)
    s.add(y)
s = list(s)
s.sort()
Z = dict()
for i, ss in enumerate(s):
    Z[ss] = i
xy = []
for x, y in XY:
    xx, yy = Z[x], Z[y]
    xy.append((xx, yy))
xy.sort()
L = len(Z)
ST = SegTree(op, e, L)
tmp = []
bk_x = -1
P = 0
Q = 0
for x, y in xy:
    if x>bk_x:
        for t in tmp:
            ST.set(t, ST.get(t)+1)
        tmp = []
    yminus = ST.prod(0, y)
    P += yminus
    yplus = ST.prod(y+1, L)
    Q += yplus
    tmp.append(y)
    bk_x = x
X = dict()
Y = dict()
for x, y in XY:
    if x not in X:
        X[x] = 1
    else:
        X[x] += 1
    if y not in Y:
        Y[y] = 1
    else:
        Y[y] += 1
R = 0
S = 0
cnt = 0
for key in X:
    cnt += X[key]*(X[key]-1)//2
R = N*(N-1)//2 - cnt
cnt = 0
for key in Y:
    cnt += Y[key]*(Y[key]-1)//2
S = N*(N-1)//2 - cnt

ans = (P-Q) / math.sqrt(R*S)
print(ans)