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.Globalization;
using System.Numerics;

class Class3
{
    static void Main(string[] args)
    {
        int N = int.Parse(Console.ReadLine());

        int totalCnt = 0;
        for (int i = 1; i < N; i++)
        {
            int AB = N - i;

            if (AB == 1)
            {
                totalCnt++;
                continue;
            }
            double max = Math.Sqrt(AB);
            int cnt = 0;
            for (int j = 1; j < max; j++)
            {
                if(AB % j == 0)
                {
                    cnt++;
                }
            }

            totalCnt += cnt * 2;

            if (max - Math.Floor(max) == 0)
            {
                totalCnt++;
            }

        }

        Console.WriteLine(totalCnt);


    }
}