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)

// C - A x B + C
#include <bits/stdc++.h>
using namespace std;
using ll = int64_t;
#define rp(i,s,e) for(ll i=(ll)(s);i<(ll)(e);++i)

int main(){
	ll n; cin>>n;
	ll cnt = 0;
	rp(a, 1, n){
		ll bn = (n-1) / a;
		rp(b, 1, bn+1){
			ll c = n - a*b;
			if(c <= 0) break;
			cnt++;
		}
	}
	cout<< cnt <<endl;
}