N = gets.to_i
X, Y = N.times.map{gets.split.map(&:to_i)}.transpose

def route(i, j)
  Math.sqrt((X[i] - X[j]) ** 2 + (Y[i] - Y[j]) ** 2)
end

sum = 0
N.times do |i|
  c = Array.new(N){|i| i}
  c.delete(i)
  c.each do |other|
    sum += route(i, other)
  end
end

puts sum / N