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 Java8 (OpenJDK 1.8.0) (judge verdict: Accepted)

import java.util.ArrayList;
import java.util.Collections;
import java.util.PriorityQueue;
import java.util.Scanner;

public class Main {
	public static void main(String[] args) {

		Scanner in = new Scanner(System.in);
		int n = in.nextInt();
		int m = in.nextInt();
		@SuppressWarnings("unchecked") ArrayList<Integer>[] jobs = new ArrayList[m + 1]; // 遅延日数ごとに仕事をグルーピングする
		for ( int i = 0; i <= m; i++ ) {
			ArrayList<Integer> a = new ArrayList<>();
			a.add(0); // ダミー要素
			jobs[i] = a;
		}

		for ( int i = 0; i < n; i++ ) {
			int d = in.nextInt();
			int f = in.nextInt();
			if ( d > m ) continue; // 遅延日数的に候補になりえない仕事は除外
			jobs[d].add(f);
		}

		long total = 0;
		PriorityQueue<Integer> pool = new PriorityQueue<Integer>(Collections.reverseOrder()); // job候補のプール

		// m日目をゼロとして考え後退していく days:残り日数
		for ( int today = 0; today <= m; today++ ) {
			pool.addAll(jobs[today]);
			total += (long)pool.poll();
		}

		System.out.println(total);
		in.close();
	}
}