N = gets.to_i
xys = N.times.map { gets.split.map(&:to_i) }

def dfs(cand, pos)
  return 0 if cand.empty?
  sum = 0.to_f
  x1, y1 = pos
  cand.each do |pos2|
    tmp = cand.dup
    tmp.delete(pos2)
    x2, y2 = pos2
    d = Math.sqrt((x1 - x2) ** 2 + (y1 - y2) ** 2)
    sum += d * (1..([cand.size - 1, 1].max)).reduce(:*)
    sum += dfs(tmp, pos2)
  end
  sum
end

sum = 0.to_f
xys.each do |pos|
  tmp = xys.dup
  tmp.delete(pos)
  sum += dfs(tmp, pos)
end
ans = sum / (1..N).reduce(:*)
puts ans
