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: Time Limit Exceeded)

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

namespace ABC179
{
    class Program
    {
        static void Main(string[] args)
        {
            var N = int.Parse(Console.ReadLine());
            int ans = 0;
            IEnumerable<int> divisors;

            for (int i = 1; i < N; i++)
            {
                divisors = GetDivisors(i);
                ans+=divisors.Count();
            }

            System.Console.WriteLine(ans);
        }

        public static IEnumerable<int> GetDivisors(int num)
        {
            if (num < 1) yield break;

            for (int i = 1; i * i <= num; i++)
            {
                if (num % i == 0)
                {
                    yield return i;
                    if (i * i != num) yield return (num / i);
                }
            }
        }
    }
}