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 c
{
    class Program
    {
        static void Main(string[] args)
        {
            var n = int.Parse(Console.ReadLine());
            int count = 0;

            // a*b+c=nを満たすa,b,cはa,bが決まれば必ず1つに定まる(a*b<nの場合)

            for (int a = 1; a < n; a++) 
            {
                for (int b = 1; b < n; b++) 
                {
                    if (a * b < n) count++;
                    else if(a * b >= n) break;
                }
            }

            Console.WriteLine(count);
        }
    }
}