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 AtCoderBiginnerContest179
{
	class Program
	{
		static void Main(string[] args)
		{
			var input = int.Parse(Console.ReadLine());
			var target = input % 2 == 0 ? input / 2 - 1 : input / 2;

			var count = 0;
			for (var i = 1; i <= target; i++)
			{
				if(input % i == 0)
				{
					count += input / i - 1;
				}
				else
				{
					count += input / i;

				}
			}

			count += input - target - 1;

			Console.WriteLine(count);
		}
	}
}