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>

int main(void)
{
    unsigned long int N;
    scanf("%lu", &N);
    
    unsigned long int cnt=0;
    unsigned long int mid;

    if((N%2)==0) {
        mid = N/2;
    } else {
        mid = N/2 + 1;
    }

    cnt=(N-1) - mid + 1;

    for(int i=1; i<=(mid-1); i++) {
        if((N%i)==0) {
            cnt+=((N/i)-1);
        } else {
            cnt+=(N/i);
        }
    }

    printf("%lu", cnt);
}