N = gets.to_i
xy = []

N.times {
  x, y = gets.split.map(&:to_i)
  xy << [x, y]
}

total = 0.0
pattern = 1
(N-2).times do |i|
  pattern *= i+1
end
pattern *= N-1
# puts "pattern=#{pattern}"

(N-1).times do |i|
  (i+1).upto(N-1) do |j|
    # puts "i=#{i} j=#{j}"
    calc = (xy[i][0] - xy[j][0]) ** 2 + (xy[i][1] - xy[j][1]) ** 2
    # puts "inside sqrt=#{calc}"
    dist = Math.sqrt(calc) *2
    total += dist * pattern
    # puts "total = #{total}"
  end
end

puts total/(pattern * N)
