def distance(a,b)
  Math.sqrt(((a[0] - b[0]) ** 2) + ((a[1] - b[1]) ** 2))
end

n = gets.chomp.to_i
m = []
n.times {|i| m.push(gets.chomp.split(' ').map{|i| i.to_i}) }

order_list = []
_o = []
n.times{|i| _o.push(i)}
_o.permutation(n){|a| order_list.push(a)}

total_distance = 0.0
order_list.each do |order|
  0.upto(n) do |i|
    break if i+1 >= n
    total_distance += (distance(m[order[i]], m[order[i+1]]))
  end
end
puts total_distance / order_list.length
