n=int(input())
p=[tuple(map(int,input().split())) for i in range(n)]

P=0
B=448
st1=[0]*B*B
st2=[0]*B
z=sorted(set([y for x,y in p]))
d={v:i for i,v in enumerate(z)}
p.sort(key=lambda a:(-a[0],a[1]))
for x,y in p:
  l=d[y]
  for i in range(l+1,((l+1)//B)*B+B):
    P+=st1[i]
  for i in range((l+1)//B+1,B):
    P+=st2[i]
  st1[l]+=1
  st2[l//B]+=1

Q=0
B=448
st1=[0]*B*B
st2=[0]*B
z=sorted(set([y for x,y in p]))
d={v:i for i,v in enumerate(z)}
p.sort(key=lambda a:(-a[0],-a[1]))
for x,y in p:
  l=d[y]
  for i in range(((l-1)//B)*B,l-1+1):
    Q+=st1[i]
  for i in range((l-1)//B):
    Q+=st2[i]
  st1[l]+=1
  st2[l//B]+=1

R=0
z=sorted(set([x for x,y in p]))
d={v:i for i,v in enumerate(z)}
c=[0]*len(z)
for x,y in p:
  c[d[x]]+=1
for i in range(len(z)):
  R+=c[i]*(n-c[i])
R//=2

S=0
z=sorted(set([y for x,y in p]))
d={v:i for i,v in enumerate(z)}
c=[0]*len(z)
for x,y in p:
  c[d[y]]+=1
for i in range(len(z)):
  S+=c[i]*(n-c[i])
S//=2

print((P-Q)/(R*S)**0.5)