N=gets.to_i
as=N.times.map{gets.split.map(&:to_i)}
sum=0
as.permutation(N) do |es|
    cnt=0
    (N-1).times do |i|
        cnt+=((es[i][0]-es[i+1][0])**2+(es[i][1]-es[i+1][1])**2)**(1/2.0)
    end
    sum+=cnt
end
puts sum/(1..N).to_a.inject(&:*)