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 = fgets(STDIN);
$array = [];

$a=1;
$b=1;
$ans=0;

while($a*$b<$n){
    while($a*$b<$n){
        $ans++;
        $b++;
    }
    $a++;
    $b=1;
}

echo $ans."\n";
// print_r($array);