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;
using System.Collections.Generic;
using System.ComponentModel;
using System.ComponentModel.Design;
using System.IO.Compression;
using System.Linq;
using System.Numerics;
using System.Runtime.CompilerServices;
using System.Security.Cryptography.X509Certificates;
using System.Text;
using System.Text.RegularExpressions;
using Console = System.Console;

namespace Atcoder
{

    class Program
    {
        private const long COMDIV = 1000000007;

        static void Main(string[] args)
        {
            var N = long.Parse(Console.ReadLine());
            int ret = 0;
            for (long a = 1; a <= 1000000; a++)
            {
                for (long b = a; b <= 1000000; b++)
                {
                    var c = N - a * b;
                    if (c > 0)
                    {
                        if (a == b)
                        {
                            ret++;
                        }
                        else
                        {
                            ret+=2;
                        }
                    }
                    else
                    {
                        break;
                    }
                }
            }

            Console.WriteLine(ret);
        }
    }
}