Problem:
Score : 300 points

Problem Statement

Given is a positive integer N.
How many tuples (A,B,C) of positive integers satisfy A \times B + C = N?

Constraints

2 \leq N \leq 10^6

All values in input are integers.

Input

Input is given from Standard Input in the following format:

N

Output

Print the answer.

Sample Input 1

3

Sample Output 1

3

There are 3 tuples of integers that satisfy A \times B + C = 3: (A, B, C) = (1, 1, 2), (1, 2, 1), (2, 1, 1).

Sample Input 2

100

Sample Output 2

473

Sample Input 3

1000000

Sample Output 3

13969985

Solution in Ruby (2.7.1) (judge verdict: Accepted)

$n = gets.chomp.to_i
$array = Array.new($n + 1, 0)

($n).times do |i|
  if i > 0
    $ii = i
    while $ii < $n
      $array[$ii] += 1
      $ii += i
    end
  end
end

$ans = 0
$array.each do |i|
  $ans += i
end
puts $ans