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.RegularExpressions;
using static kyoupuro.Input;
using static kyoupuro.MyMath;

namespace kyoupuro {

    class MainClass {
        
        public static void Main() {
            var N = NextInt();
            var K = NextInt();
            long count = 0;

            for (int mod = K + 1; mod <= N; mod++) {

                // a % b にて a < bの時
                count += mod - K - (K == 0 ? 1 : 0);

                // 上記以外
                if(mod + K <= N) {
                    count += (N / mod - 1) * (mod - K) + Math.Max(0, N % mod - K + 1);
                }
            }

            Console.WriteLine(count);
        }
    }

    class MyMath {
        static long Gcd(long a, long b) {
            return b == 0 ? a : Gcd(b, a % b);
        }
    }

    class Input {
        static IEnumerator<string> enumerator = new string[] { }.AsEnumerable().GetEnumerator();

        public static string Line() {
            return Console.ReadLine();
        }

        public static int NextInt() {
            while (!enumerator.MoveNext()) {
                enumerator = StrArr().AsEnumerable().GetEnumerator();
            }
            return int.Parse(enumerator.Current);
        }

        public static long NextLong() {
            while (!enumerator.MoveNext()) {
                enumerator = StrArr().AsEnumerable().GetEnumerator();
            }
            return long.Parse(enumerator.Current);
        }

        public static string[] StrArr() {
            return Line().Split(' ');
        }

        public static List<int> IntList() {
            return StrArr().Select(int.Parse).ToList();
        }

        public static List<long> LongList() {
            return StrArr().Select(long.Parse).ToList();
        }

        public static void Skip(int line = 1) {
            enumerator.Reset();
            for (int i = 0; i < line; i++) Console.ReadLine();
        }

        public static void Reset() {
            enumerator.Reset();
        }
    }
}