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)

use std::collections::BinaryHeap;

fn main() {
    let (n, m) = {
        let v = read_vec::<usize>();
        (v[0], v[1])
    };

    // 仕事の読み込み
    let mut works = vec![vec![]; m + 1];
    for _ in 0..n {
        let (a, b) = {
            let v = read_vec::<usize>();
            (v[0], v[1])
        };
        if a <= m { works[a].push(b); }
    }

    let mut heap = BinaryHeap::new();
    let mut reword: usize = 0;
    for i in 1..m+1 {
        // heapにi日後に報酬がもらえるものを加える
        for b in works[i].iter() {
            heap.push(b);
        }

        if let Some(max) = heap.pop() {
            reword += *max;
        }
    }

    println!("{}", reword);
}

#[allow(dead_code)]
fn read<T: std::str::FromStr>() -> T  {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).unwrap();
    s.trim().parse::<T>().ok().unwrap()
}

#[allow(dead_code)]
fn read_vec<T: std::str::FromStr>() -> Vec<T> {
    read::<String>().split_whitespace()
    .map(|e| e.parse().ok().unwrap())
    .collect()
}