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.Linq;//リストの使用
using System.Collections.Generic;
class Program
{
	static void Main()
	{
		string[] input = Console.ReadLine().Split(' ');//Splitで区切り文字を指定して複数個受け取る。
		long n = long.Parse(input[0]);
		long k = long.Parse(input[1]);
		long answer = 0;
    long maxDiv = 0;

    for(long divideNum = k+1; divideNum <= n; divideNum++)
    {
      if(n % divideNum != 0) maxDiv = n+divideNum-(n%divideNum);
      else maxDiv = n;
      answer += Math.Max(0,(maxDiv/divideNum-1) * (divideNum-k));
      //Console.WriteLine(answer);
      answer += Math.Max(0, n-k-(maxDiv-divideNum)+1);
      //Console.WriteLine(answer);
    }

    if(k == 0) Console.WriteLine(n*n);
		else Console.WriteLine(answer);//WriteLineをWriteとすると、改行なしで出力。
	}
}