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 Rust (1.42.0) (judge verdict: Accepted)

use proconio::fastout;
use whiteread::parse_line;

#[fastout]
fn main() {
    let (n, mut k): (usize, usize) = parse_line().unwrap();

    let mut ret = 0;

    if k == 0 {
        for b in 1..=n {
            ret += n / b;
        }

        k += 1;
    }

    for b in (k + 1)..=n {
        ret += ((n - k) / b + 1) * (b - k);
        if (n - k) % b + 1 <= b - k - 1 {
            ret -= b - k - (n - k) % b - 1;
        }
    }
    println!("{}", ret);
}