n = gets.to_i
xys = []
n.times do
  xys << gets.strip.split.map(&:to_i)
end

sum_distance = 0

xys.permutation(n) do |xy_root|
  0.upto(n-2) do |i|
    sum_distance += Math.sqrt((xy_root[i][0] - xy_root[i+1][0]) ** 2 + (xy_root[i][1] - xy_root[i+1][1]) ** 2)
  end
end
ans = sum_distance / xys.permutation(n).size
puts printf("%.10f", ans)