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: Time Limit Exceeded)

#include<stdio.h>

int main(void){
    int N;
    int a=1,b=1,c=1;
    int flag=0;

    int x = scanf("%d",&N);


    for(c=1;c<N;c++){
        for(a=1;a<N;a++){
            for(b=1;a*b+c<=N;b++){
                //printf("(a,b,c,N)=(%d,%d,%d,%d)\n",a,b,c,N);
                //printf("a*b+c=%d\n",a*b+c);
                if(a*b+c==N){
                    flag++;
                }
            }
        }
    }

    printf("%d\n",flag);
    return 0;
}