Problem:
Score : 400 points

Problem Statement

There are N one-off jobs available. If you take the i-th job and complete it, you will earn the reward of B_i after A_i days from the day you do it.

You can take and complete at most one of these jobs in a day.

However, you cannot retake a job that you have already done.

Find the maximum total reward that you can earn no later than M days from today.

You can already start working today.

Constraints

All values in input are integers.

1 \leq N \leq 10^5

1 \leq M \leq 10^5

1 \leq A_i \leq 10^5

1 \leq B_i \leq 10^4

Input

Input is given from Standard Input in the following format:

N M
A_1 B_1
A_2 B_2
\vdots
A_N B_N

Output

Print the maximum total reward that you can earn no later than M days from today.

Sample Input 1

3 4
4 3
4 1
2 2

Sample Output 1

5

You can earn the total reward of 5 by taking the jobs as follows:

Take and complete the first job today. You will earn the reward of 3 after four days from today.

Take and complete the third job tomorrow. You will earn the reward of 2 after two days from tomorrow, that is, after three days from today.

Sample Input 2

5 3
1 2
1 3
1 4
2 1
2 3

Sample Output 2

10

Sample Input 3

1 1
2 1

Sample Output 3

0

Solution in Rust (1.15.1) (judge verdict: Accepted)

#[allow(unused_macros)]
macro_rules! input {
    ( $($t:ty),* ) => {{
        let mut s = String::new();
        std::io::stdin().read_line(&mut s);
        let mut splits = s.trim().split_whitespace();
        ($( { splits.next().unwrap().parse::<$t>().unwrap() },)*)
    }}
}

use std::cmp::Ordering;

#[derive(Eq)]
struct Work {
    day: u32,
    reward: u32,
}

impl Ord for Work {
    fn cmp(&self, other: &Self) -> Ordering {
        self.reward.cmp(&other.reward)
    }
}

impl PartialOrd for Work {
    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
        Some(self.cmp(other))
    }
}

impl PartialEq for Work {
    fn eq(&self, other: &Self) -> bool {
        self.reward == other.reward
    }
}

use std::collections::BinaryHeap;

#[allow(unused_must_use)]
#[allow(unused_variables)]
fn solve() {

    let (n, m) = input!(usize, usize);

    let mut works = Vec::with_capacity(n);
    let mut pq = BinaryHeap::new();

    for i in 0..n {
        let (a, b) = input!(u32, u32);
        works.push( Work { day: a, reward: b } );
    }

    works.sort_by(|a, b| b.day.cmp(&a.day));

    let mut total_reward = 0;

    for day in 1..(m+1) {
        while works.len() > 0 && works[works.len() - 1].day <= day as u32 {
            if let Some(work) = works.pop() {
                pq.push(work);
            }
        }

        if let Some(work) = pq.pop() {
            total_reward += work.reward;
        }
    }
    
    println!("{}", total_reward);
}

fn main() {
    solve();
}