n = gets.to_i
towns = (0..n).to_a
array = []
n.times do
x,y = gets.split(" ").map {|v| v.to_i}
array << [x,y]
end
d = 0
c_towns = towns.clone
while c_towns.size >= 2
  a = c_towns.pop
  for b in c_towns
    dis = (towns[a][0] - towns[b][0]) ** 2 + (towns[a][1] - towns[b][1]) ** 2
    d += Math.sqrt(dis)
  end
end
puts d / n - 1