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 <iostream>

using namespace std;
int counting(int x) {
    int ans = 0;
    for (int i=1; i * i <= x; i++) {
        if (x % i == 0) {
            if (i * i == x) ans++;
            else ans += 2;
        }
    }
    return ans;
}
int main() {
    int N;
    cin >> N;
    int ans = 0;
    for (int nc = 1; nc <= N-1; nc++) {
        ans += counting(nc);
    }
    cout << ans << endl;
    return 0;
}