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;
#define w(x) int x;cin>>x;while(x--)
#define ll long long
#define pb push_back
#define fast ios_base::sync_with_stdio(false);cin.tie(NULL)
#define vi vector<int>
#define fr(a,b) for(int i=a;i<=b;i++)
#define inf 1e18
#define mod 1000000007
int main() {
/*#ifndef ONLINE_JUDGE
	freopen("input.txt", "r",stdin);
	freopen("output.txt", "w",stdout);
#endif*/
	int n;
	cin>>n;
	ll ans=0;
	for(int i=1;i<=n;i++)
	{
		int k=n/i;
		if(n%i==0)
			ans+=(k-1);
		else
			ans+=k;
	}
	cout<<ans<<"\n";
    return 0;
}