N = gets.to_i
xs = N.times.map{gets.split.map(&:to_i)}
ans = 0
xs.permutation(N) do |es|
    es.each_cons(2) do |a,b|
        ans += ((a[0]-b[0])**2 + (a[1]-b[1])**2)**(0.5)
    end
end
puts ans/((1..N).to_a.inject(&:*))
