n = gets.chomp("").to_i
x = []
y = []
n.times do |i|
  x[i],y[i] = gets.split.map(&:to_i)
end

ans = 0.0
(n-1).times do |i|
  (n-i-1).times do |t|
    yoko = (x[i] - x[i+1+t])*(x[i] - x[i+1+t])
    tate = (y[i] - y[i+1+t])*(y[i] - y[i+1+t])
    ans += (yoko+tate)**(1/2.0)
  end
end

puts (ans*2)/n