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)

/// 15:52 20/09/20
/// Author: t_bone_289 (An Immature Programmer.... I'm still developing !!)

#include<bits/stdc++.h>

using namespace std;
#define endl '\n'
typedef int ll;

ll getFactors(ll n) {
    ll ans = 0;
    for (ll i = 1; i * i <= n; i++) {
        if (n % i == 0) {
            ans += 2;
            if (i * i == n) {
                ans--;
            }
        }
    }
    return ans;
}

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);
    ll n;
    cin >> n;
    ll ans = 0;
    for (ll i = 1; i < n; i++) {
        ll d = n - i;
        ans += getFactors(d);
    }
    cout << ans;
    return 0;
}