n=gets.to_i
x=Array.new(n,0)
y=Array.new(n,0)
n.times do |i|
  x[i],y[i]=gets.split.map(&:to_f)
end

dist=0
sum=0
count=0
[*1..n].permutation.each do |p|
  dist=0
  (n-1).times do |i|
    dist+=((x[p[i+1]-1]-x[p[i]-1])**2+(y[p[i+1]-1]-y[p[i]-1])**2)**0.5
  end
  sum+=dist
  count+=1
end
ave=sum/count
puts sprintf("%.6f", ave)
