include Math

N = gets.to_i
town = (0..N-1).to_a
x, y = N.times.map{gets.split.map(&:to_i)}.transpose
sum = 0

town.combination(2) do |i, j|
  sum += sqrt((x[i]-x[j])**2 + (y[i]-y[j])**2)
end

ans = 2*sum / N

puts sprintf("%.10f", ans)