n = gets.to_i
x = []
y = []
for i in 0...n do
  x[i], y[i] = gets.split.map(&:to_i)
end
perm = (0...n).to_a.permutation.to_a
sum = 0.0
perm.each do |a|
  for i in 0...(a.length - 1) do
    d = Math.sqrt((x[a[i]] - x[a[i + 1]]) ** 2 + (y[a[i]] - y[a[i + 1]]) ** 2)
    sum += d
  end
end
puts sum / perm.length
