def dis(x1, y1, x2, y2)
  return ( (x2 - x1)**2 + (y2 - y1)**2 )**(1/2.0)
end

n = gets.to_i
x, y = n.times.map{gets.split.map(&:to_i)}.transpose

total = 0
n.times do |i1|
  n.times do |i2|
    total += dis(x[i1], y[i1], x[i2], y[i2])
  end
end
puts total / n