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.IO;
using System.Linq;
using System.Collections.Generic;
using System.Text;
using System.Numerics;

namespace abc179
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.SetOut(new StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false });
            if (File.Exists("input.txt"))
            {
                Console.SetIn(new StreamReader("input.txt"));
            }
            solve_abc179C();
            Console.Out.Flush();
        }
        public static void solve_abc179C()
        {
            int n = Convert.ToInt32(Console.ReadLine());
            int cnt = 0;
            for (int i = 1; i <= 1000000; i++)
            {
                for (int j = 1; j <= 1000000; j++)
                {
                    if (i * j < n)
                    {
                        cnt++;
                        //Console.WriteLine("{0} * {1} = {2}, {3} - {2} = {4}", i, j, i * j, n, n - (i * j));
                    }
                    else
                    {
                        break;
                    }
                }
            }
            Console.WriteLine(cnt);
        }

        public static void solve_abc179B()
        {
            int t = Convert.ToInt32(Console.ReadLine());
            int cnt = 0;
            int max = 0;
            while (t-- > 0)
            {
                int[] d = Array.ConvertAll(Console.ReadLine().Split(), int.Parse);
                if (d[0] == d[1])
                {
                    cnt++;
                }
                else
                {
                    cnt = 0;
                }
                max = Math.Max(max, cnt);
            }
            Console.WriteLine(max >= 3 ? "Yes" : "No");
        }

        public static void solve_abc179A()
        {
            string s = Console.ReadLine();
            Console.WriteLine(s.EndsWith('s') ? s + "es" : s + "s");
        }
    }
}