n = gets.chomp.to_i
pos_list = []
distance_list = []

n.times do
  pos_list << gets.chomp.split(' ').map(&:to_i)
end

def distance(x1,y1,x2,y2)
  return Math.sqrt((x1-x2)**2 + (y1-y2)**2)
end

for i in 0..n-1
  for j in 0..n-1
    next if (i == j)
    distance_list[i] = 0 if (distance_list[i].nil?) 
    distance_list[i] += distance(pos_list[i][0], pos_list[i][1], pos_list[j][0], pos_list[j][1])
  end
end

sum = 0
distance_list.each do |d|
  sum += d
end

puts sum / distance_list.size
