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 PHP (7.4.4) (judge verdict: Accepted)

<?php
$n=trim(fgets(STDIN));
$a=1;
$ans=0;
while($a<=ceil($n/2)){
  $b=$a;
  while($a*$b<$n){
    $ans+=2;
    if($a==$b){
      --$ans;
    }
    ++$b;
  }
  ++$a;
}
echo $ans;