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;
using ll = long long;

vector<ll> divisor(ll n){
    vector<ll> vec;
    for(ll i=1;i*i<= n;i++){
        if(n%i==0){
            vec.push_back(i);
            if(i*i!=n) vec.push_back(n/i);
        }
    }
    sort(vec.begin(), vec.end());
    return vec;
}

int main(void){
    int n;
    cin >> n;
    
    ll ans = 0;
    for(int a=1;a<=n;a++){
        ans += (n-1)/a;
    }
    cout << ans << endl;

    return 0;
}