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: Wrong Answer)

#define DEBUG

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

using E = System.Linq.Enumerable;

public class Program
{
    public static void Main()
    {
        var n = int.Parse(Console.ReadLine());

        var k = 0;
        for (int c = 1; c < n; ++c)
        {
            int a = 1;
            for (; a * a <= n - c; ++a)
            {
                if ((n - c) % a != 0)
                {
                    continue;
                }

                k += 2;
            }
            if (a * a == n - c)
            {
                k += 1;
            }
        }

        Console.WriteLine(k);
    }
}