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

namespace Atcoder {
    class Program {
        static void Main(string[] args) {
            //var N = int.Parse(Console.ReadLine());
            //var inputList = new List<int>();
            //foreach (var i in Enumerable.Range(0, N))
            //    inputList.Add(int.Parse(Console.ReadLine()));


            var NK = Console.ReadLine().Split(' ');
            var N = int.Parse(NK.First());
            var K = int.Parse(NK.Last());

            long count = 0;
            for (int b = 1; b <= N; b++) {
                count += (max(b - K, 0) * (N / b) + max((N % b) - K + 1, 0));
                if (K == 0)
                    count -= 1;
            }
            Console.WriteLine(count);
            return;
        }

        static long max(long left, long right) {
            if (left >= right)
                return left;
            return right;
        }
    }
}