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: Accepted)

using System;
using System.Linq;

namespace arc091_b
{
    class Program
    {
        static void Main(string[] args)
        {
            int[] x = Console.ReadLine().Split().Select(int.Parse).ToArray();
            long ret = 0;
            for (int i = x[1] + 1; i <= x[0]; i++)
            {
                int a = (x[0] - (i - 1)) / i + 1;
                ret += a * (i - x[1]);
                if ((x[0] - (i - 1)) / i != (x[0] - x[1]) / i)
                {
                    ret += (x[0] - x[1]) % i + 1;
                }
            }
            if (x[1] == 0) ret -= x[0];
            Console.WriteLine(ret);
        }
    }
}