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

arr = (1..n).to_a

total = 0.0
t = 0
arr.permutation(n) do |a|
  t += 1
  0.upto(n-2) do |i|
    total += Math.sqrt((x[a[i]]-x[a[i+1]])**2 + (y[a[i]]-y[a[i+1]])**2)
  end
end
p total / t
