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)

// AtCoder :: Beginner Contest 179 :: C - A x B + C
// https://atcoder.jp/contests/abc179/tasks/abc179_c

#include <bits/stdc++.h>

using namespace std;
using ll = long long;

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    ll N;
    cin >> N;
    ll soln{0};
    for (ll a{1}; a < N; a++) {
        for (ll b{1}; a * b < N; b++) {
            soln++;
        }
    }
    cout << soln << endl;
    return EXIT_SUCCESS;
}