Problem:
Score : 400 points

Problem Statement

Takahashi had a pair of two positive integers not exceeding N, (a,b), which he has forgotten.
He remembers that the remainder of a divided by b was greater than or equal to K.
Find the number of possible pairs that he may have had.

Constraints

1 \leq N \leq 10^5

0 \leq K \leq N-1

All input values are integers.

Input

Input is given from Standard Input in the following format:

N K

Output

Print the number of possible pairs that he may have had.

Sample Input 1

5 2

Sample Output 1

7

There are seven possible pairs: (2,3),(5,3),(2,4),(3,4),(2,5),(3,5) and (4,5).

Sample Input 2

10 0

Sample Output 2

100

Sample Input 3

31415 9265

Sample Output 3

287927211

Solution in C# (Mono 4.6.2.0) (judge verdict: Wrong Answer)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Atcoder2
{
    class Program
    {
        static void Main(string[] args)
        {
            string[] str = Console.ReadLine().Split(' ');

            int N = int.Parse(str[0]);
            int K = int.Parse(str[1]);

            long ans = 0;

            ans += ((N - N / 2 - 1) * (N - N / 2)) / 2;

            for(int i = K; i < (N/2)+1; i++)
            {
                for(int b = i + 1; b <= N; b++)
                {
                    int plus = ((N - i) / b) + 1;
                    ans += plus;
                    if (plus == 1)
                    {
                        ans += N - b;
                        break;
                    }
                }
            }
            if (K == 0)
            {
                ans -= N;
            }

            Console.WriteLine(ans);
        }
    }
}