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 Java8 (OpenJDK 1.8.0) (judge verdict: Accepted)

import java.util.*;

import static java.lang.System.*;

//https://atcoder.jp/contests/abc090/tasks/arc091_b
//a%b>=Kの組み合わせ。bを固定して考えるとO(n)
public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        int K = sc.nextInt();

        solver(N, K);
    }

    public static void solver(int N, int K) {
        long ans = 0;

        for (int b = 1; b <= N; b++) {
            // 0<= a=pb+r <=N
            int p = N / b;
            int r = N % b;
            ans += p*Math.max(b-K, 0);
            ans += Math.max(r-K+1, 0);
        }
        // 例外：K=0の時は、a=0を含んでしまうため、カウントしない
        if(K==0) {
            ans -= N;
        }
        out.println(ans);
    }
}