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;
class Program
{
    static void Main(string[] args)
    {
        string a = Console.ReadLine(); //標準入力


        int N = int.Parse(a);

        decimal count = 0;

        for (int i = 1; i<N;i++)
        {
            if (N % i == 0)
            {
                count += (decimal)(N / i - 1);
            }
            else
            {
                count += (decimal)(N / i);
            }
        }

        Console.WriteLine("{0}", count);
    }

}