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 {
    ($x:expr) => {
        let mut buf = String::new();
        std::io::stdin().read_line(&mut buf).unwrap();
        $x = buf.trim().parse().unwrap();
    };

    ($x:expr, $($y:expr), +) => {
        let mut buf = String::new();
        std::io::stdin().read_line(&mut buf).ok();
        input_iter!(buf.split_whitespace(), $x, $($y), +);
    };

    ($v:expr; $size:expr) => {
        let mut buf = String::new();
        std::io::stdin().read_line(&mut buf).ok();
        for v in buf.split_whitespace() {
            $v.push(v.parse().unwrap());
        }
    };

    ($v:expr; $h_size:expr, $w_size:expr) => {
        for _ in 0..$h_size {
            let mut part_vec = Vec::new();
            let mut buf = String::new();
            std::io::stdin().read_line(&mut buf).ok();
            for v in buf.split_whitespace() {
                part_vec.push(v.parse().unwrap());
            }
            $v.push(part_vec);
        }
    };
}

#[allow(unused_macros)]
macro_rules! input_iter {
    ($iter:expr, $x:expr) => {
        $x = $iter.next().unwrap().parse().unwrap();
    };

    ($iter:expr, $x:expr, $($y:expr), +) => {
        let mut iter = $iter;
        $x = iter.next().unwrap().parse().unwrap();
        input_iter!(iter, $($y), *);
    };
}

use std::collections::*;
use std::collections::hash_map::Entry::*;

fn main() {
    let (n, m): (usize, usize);
    input!(n, m);
    let mut works = vec![vec![]; m + 1];
    for _ in 0..n {
        let (a, b): (usize, usize);
        input!(a, b);
        if a <= m {
            works[a].push(b);
        }
    }
    let mut candidates = BinaryHeap::new();
    let mut ans = 0;
    for i in 1..(m + 1) {
        for work in &works[i] {
            candidates.push(work);
        }
        ans += match candidates.pop(){
            Some(x) => *x,
            None => 0,
        };
    }
    println!("{}", ans);
}