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<bits/stdc++.h>
using namespace std;
int n, a, b;
long long s;
int main()
{
	cin>>n;
	for(a=1; a<=n/2; a++)
	{
		for(b=a; b<=n/a; b++)
		{
			if(a*b<n)
			{
				s++;
				if(a!=b)
				{
					s++;
				}
			}
			else
			{
				break;
			}
		}
	}
	cout<<s;
	return 0;
}