n = gets.to_i
points = readlines.map{|s| s.split.map(&:to_i)}

def d(p1, p2)
  Math.sqrt((p1[0] - p2[0]) ** 2 + (p1[1] - p2[1]) ** 2)
end

g = Array.new(n) {Array.new(n){0}}

(0...n).each do |i|
  (0...n).each do |j|
    next if i == j
    g[i][j] = d(points[i], points[j])
  end
end

puts g.flatten.reduce(:+) / n