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 ABC
{
    class Program
    {
        static void Main(string[] args)
        {
            var n = Cin.Int;
            var ans = 0L;
            for(int c = 1; c < n; c++)
            {
                var m = n - c;
                ans += Tool.DivsCnt(m);
            }
            Console.WriteLine(ans);
        }
    }

    static class Cin
    {
        public static string[] ReadSplit => Console.ReadLine().Split();
        public static int[] ReadSplitInt => Console.ReadLine().Split().Select(int.Parse).ToArray();
        public static long[] ReadSplitLong => Console.ReadLine().Split().Select(long.Parse).ToArray();
        public static string Str => Console.ReadLine();
        public static int Int => int.Parse(Console.ReadLine());
        public static long Long => long.Parse(Console.ReadLine());
        public static double Double => double.Parse(Console.ReadLine());
        public static T Conv<T>(string input)
        {
            return (T)Convert.ChangeType(input, typeof(T));
        }
        public static void Scanf<T>(out T a) => a = Conv<T>(Console.ReadLine());
        public static void Scanf<T,U>(out T a, out U b)
        {
            var q = ReadSplit; a = Conv<T>(q[0]); b = Conv<U>(q[1]);
        }
        public static void Scanf<T, U, V>(out T a, out U b, out V c)
        {
            var q = ReadSplit; a = Conv<T>(q[0]); b = Conv<U>(q[1]); c = Conv<V>(q[2]);
        }
        public static void Scanf<T, U, V, W>(out T a, out U b, out V c, out W d)
        {
            var q = ReadSplit; a = Conv<T>(q[0]); b = Conv<U>(q[1]); c = Conv<V>(q[2]); d = Conv<W>(q[3]);
        }
    }
    static class Cout
    {
        public static void OutL(object s) => Console.WriteLine(s);
    }
    static class Tool
    {
        public static long Gcd(long a, long b)
        {
            if (a % b == 0)
            {
                return b;
            }
            else
            {
                return Gcd(b, a % b);
            }
        }
        public static long Lcm(long a, long b) => a * b / Gcd(a, b);

        public static long[] PrimeFac(long n)
        {
            var result = new long[n + 1];
            for(int i = 2; i * i <= n; i++)
            {
                if (n % i != 0)
                {
                    continue;
                }

                var cnt = 0;
                while(n % i == 0)
                {
                    cnt++;
                    n /= i;
                }
                result[i] = cnt;
            }
            if(n != 1)
            {
                result[n] = 1;
            }
            return result;
        }
        public static long DivsCnt(long n)
        {
            var divisorList = PrimeFac(n);
            var result = 1L;
            for(int i = 2; i <= n; i++)
            {
                result *= (divisorList[i] + 1);
            }
            return result;
        }
    }
}