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;

namespace ConsoleApp1
{
    class ABC179C
    {
        // https://atcoder.jp/contests/abc179/tasks/abc179_c
        static void Main(string[] args)
        {
            int N = Int32.Parse(Console.ReadLine());

            long result = 0;

            for (int A = 1; A < N; A++)
            {
                result += (N - 1) / A;
            }

            Console.WriteLine(result);
        }
    }
}