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 C (GCC 9.2.1) (judge verdict: Accepted)

#include <stdio.h>
#include <math.h>
int main()
{
    int n;
    int ans = 0;
    int sn;
    scanf("%d", &n);
    sn = (int)sqrt((double)(n - 1));

    for (int i = 1; i <= sn; i++)
    {
        ans += 2 * ((n - 1) / i);
    }
    ans -= sn * (sn - 1);
    ans -= sn;
    printf("%d", ans);
    return 0;
}