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# (.NET Core 3.1.201) (judge verdict: Accepted)

using System;
using System.Collections.Generic;
using System.Linq;

class C
{
    static int[] Read() => Console.ReadLine().Split().Select(int.Parse).ToArray();
    static void Main()
    {
        var n = int.Parse(Console.ReadLine());
        int ans = 0;
        for (int a = 1, b = 1; b < n; b++)
        {
            while (a * b < n)
            {
                ans++;
                a++;
            }
            a = 1;
        }
        Console.WriteLine(ans);
    }
}