$sum = 0;
$cnt = 0;

def rec(n, i, used, index, points)
  if (i == n)
    (n - 1).times do |j|
      p1 = points[index[j]]
      p2 = points[index[j + 1]]
      $sum += Math.sqrt((p1[0] - p2[0]) ** 2 + (p1[1] - p2[1]) ** 2)
    end
    $cnt += 1
    return
  end

  n.times do |j|
    if (!used[j])
      used[j] = true
      index.push(j)
      rec(n, i + 1, used, index, points)
      used[j] = false
      index.pop
    end
  end
end

N = gets.to_i
points = [];
N.times do
  points << gets.split.map(&:to_f)
end

rec(N, 0, Array.new(N, false), [], points)

puts $sum / $cnt