def solve(n, xs, ys)
  len = [*0...n].combination(2).map do |i, j|
    dx = xs[i] - xs[j]
    dy = ys[i] - ys[j]
    Math.sqrt(dx ** 2 + dy ** 2)
  end.inject(:+)
  sprintf('%.10f', len / (n / 2.0))
end

n = gets.to_i
xs = []
ys = []
n.times do |_|
  xy = gets.chomp.split.map(&:to_i)
  xs << xy.first
  ys << xy.last
end

puts solve(n, xs, ys)
