N = gets.chomp.to_i
x = []; y = []
N.times do |i|
  x[i], y[i] = gets.chomp.split(" ").map(&:to_i)
end
distance = 0
(0..(N-1)).to_a.combination(2) do |i, j|
  distance += Math.sqrt((x[i]-x[j])**2 + (y[i]-y[j])**2)
end
puts 2*distance / N