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 (1.15.1) (judge verdict: Accepted)

#![allow(unused_imports, unused_macros, dead_code)]
use std::f64::*;
use std::cmp::*;
use std::collections::*;

macro_rules! dump{
    ($($a:expr),*) => {
        #[cfg(debug_assertions)]
        eprintln!(
            concat!("{}:{}:{}: ",$(stringify!($a), " = {:?}, "),*),
            file!(), line!(), column!(), $($a),*
        );
        #[cfg(not(debug_assertions))]
        {};
    }
}

const INF: i64 = 0x3f3f3f3f3f3f3f3f;
const MOD: i64 = 1000000007;
const NOTHING: usize = std::usize::MAX;
const DX: [usize; 4] = [1, 0, std::usize::MAX, 0];
const DY: [usize; 4] = [0, 1, 0, std::usize::MAX];

fn solve<R: std::io::Read, W: std::io::Write>(io: &mut IO<R, W>) {
    let (n, k): (i64, i64) = io.read();
    let mut res: i64 = 0;
    if k == 0 {
        io.writeln(n * n);
        return;
    }
    for x in k + 1..n + 1 {
        res += n / x * (x - k);
        res += max(0, n % x - k + 1)
    }
    io.writeln(res);
}

fn main() {
    use std::io::*;
    let stdin = stdin();
    let stdin = BufReader::new(stdin.lock());
    let stdout = stdout();
    let stdout = BufWriter::new(stdout.lock());
    let mut io = IO::new(stdin, stdout);
    solve(&mut io);
    io.flush();
}

pub struct IO<R, W> {
    reader: std::io::Bytes<R>,
    writer: W,
}

impl<R: std::io::Read, W: std::io::Write> IO<R, W> {
    pub fn new(r: R, w: W) -> IO<R, W> {
        IO {
            reader: r.bytes(),
            writer: w,
        }
    }

    pub fn read<T: Read>(&mut self) -> T {
        T::read(&mut self.reader)
    }

    pub fn reads<T: Read>(&mut self, n: usize) -> Vec<T> {
        (0..n).map(|_| T::read(&mut self.reader)).collect()
    }

    pub fn write<T: std::fmt::Display>(&mut self, x: T) {
        self.writer.write_fmt(format_args!("{}", x)).unwrap();
    }
    pub fn writeln<T: std::fmt::Display>(&mut self, x: T) {
        self.writer.write_fmt(format_args!("{}\n", x)).unwrap();
    }

    pub fn writes<T: std::fmt::Display>(&mut self, xs: &Vec<T>, sep: &str) {
        let mut it = xs.iter();
        for &x in it.next().iter() {
            self.write(x);
        }
        for x in it {
            self.writer.write(sep.as_bytes()).unwrap();
            self.write(x);
        }
    }
    pub fn writesln<T: std::fmt::Display>(&mut self, xs: &Vec<T>, sep: &str) {
        self.writes(xs, sep);
        self.writer.write(b"\n").unwrap();
    }

    pub fn flush(&mut self) {
        self.writer.flush().unwrap()
    }
}

pub trait Read {
    fn read<R: std::io::Read>(bytes: &mut std::io::Bytes<R>) -> Self;
}

#[inline(always)]
pub fn is_whitespace(x: u8) -> bool {
    x < 33u8 || 126u8 < x
}

macro_rules! read_int_impl{
    ($($t:ty)*) => ($(
        impl Read for $t {
            fn read<R: std::io::Read>(bytes: &mut std::io::Bytes<R>) -> Self {
                let head: u8 = bytes
                    .map(|x| x.unwrap())
                    .skip_while(|&x| is_whitespace(x))
                    .next()
                    .unwrap();
                if head == 45u8 {
                    bytes.map(|x| x.unwrap())
                        .take_while(|&x| !is_whitespace(x))
                        .fold(0, |a, x| 10 * a - x as $t + 48)
                } else {
                    bytes.map(|x| x.unwrap())
                        .take_while(|&x| !is_whitespace(x))
                        .fold((head - 48) as $t, |a, x| 10 * a + x as $t - 48)
                }
            }
        }
    )*)
}
read_int_impl! { isize i8 i16 i32 i64 usize u8 u16 u32 u64 }

macro_rules! read_float_impl{
    ($($t:ty)*) => ($(
        impl Read for $t {
            fn read<R: std::io::Read>(bytes: &mut std::io::Bytes<R>) -> Self {
                String::read(bytes).as_str().parse::<$t>().unwrap()
            }
        }
    )*)
}
read_float_impl! { f32 f64 }

impl Read for char {
    fn read<R: std::io::Read>(bytes: &mut std::io::Bytes<R>) -> Self {
        bytes.map(|x| x.unwrap())
            .skip_while(|&x| is_whitespace(x))
            .next()
            .unwrap() as char
    }
}

impl Read for String {
    fn read<R: std::io::Read>(bytes: &mut std::io::Bytes<R>) -> Self {
        bytes.map(|x| x.unwrap())
            .skip_while(|&x| is_whitespace(x))
            .take_while(|&x| !is_whitespace(x))
            .map(|x| x as char)
            .collect()
    }
}

impl Read for Vec<char> {
    fn read<R: std::io::Read>(bytes: &mut std::io::Bytes<R>) -> Self {
        bytes.map(|x| x.unwrap())
            .skip_while(|&x| is_whitespace(x))
            .take_while(|&x| !is_whitespace(x))
            .map(|x| x as char)
            .collect()
    }
}

macro_rules! read_tuple_impl{
    ($($t:ident),+) => {
        impl<$($t: Read),*> Read for ($($t),*) {
            fn read<R: std::io::Read>(bytes: &mut std::io::Bytes<R>) -> Self {
                ($($t::read(bytes)),+)
            }
        }
    }
}
read_tuple_impl!(A, B);
read_tuple_impl!(A, B, C);
read_tuple_impl!(A, B, C, D);
read_tuple_impl!(A, B, C, D, E);