n = gets.to_i
x = []; y = []
n.times.map { |i| x[i], y[i] = gets.split.map(&:to_i) }
ans = 0.0
n.times do |i|
  n.times do |j|
    next if i == j

    ans += ((x[j] - x[i])**2 + (y[j] - y[i])**2)**0.5
  end
end
ans /= n
puts ans