class BIT:
    def __init__(self,n):
        self.n = n; self.k = [0]*(n+1)
    def a(self,i,x):
        while i<=self.n: self.k[i] += x; i += i&-i
    def s(self,i):
        t = 0
        while i>0: t += self.k[i]; i -= i&-i
        return t
n = int(input())
xy = []; t = set()
for _ in range(n):
    x,y = map(int,input().split())
    xy.append((x,y)); t |= {x,y}
d = {v:i for i,v in enumerate(sorted(t))}
m = len(t); a = [[] for _ in range(m)]; b = [0]*m
for x,y in xy: a[d[x]].append(d[y]); b[d[y]] += 1
bit = BIT(m); c = p = q = 0
for x in range(m):
    for y in a[x]: p += bit.s(y); q += c-bit.s(y+1)
    for y in a[x]: bit.a(y+1,1); c += 1
r = n*(n-1)//2-sum(len(ai)*(len(ai)-1)//2 for ai in a)
s = n*(n-1)//2-sum(bi*(bi-1)//2 for bi in b)
print((p-q)/(r*s)**0.5)