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.Linq;

namespace ProgramingLearning
{
    class Program
    {
        static void Main(string[] args)
        {
            
            var n = int.Parse(Console.ReadLine());

            var ans = 1;

            for(int i = 2; i < n; i++)
            {
                var m = i;

                //mの素因数分解
                var sum = 1;
                for (int j = 2; j <= Math.Sqrt(m); j++)
                {
                    var count = 0;
                    while(m % j == 0)
                    {
                        count++;
                        m /= j;
                    }
                    sum *= (count + 1);
                    if(m == 1)
                    {
                        break;
                    }
                }
                if(m != 1)
                {
                    sum *= 2;
                }
                ans += sum;
            }
            Console.WriteLine(ans);
        }
    }
}