n=gets.to_i
x=[]
y=[]
sum=0
n.times do |i|
  x[i],y[i]=gets.split.map &:to_f
end
a=((0...n).to_a).permutation(n).to_a
a.each do|i|
  (n-1).times do |j|
    sum+=Math.sqrt((x[i[j]]-x[i[j+1]])**2+(y[i[j]]-y[i[j+1]])**2)
  end
end
puts sum/n
