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 Java (OpenJDK 11.0.6) (judge verdict: Accepted)

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.Comparator;
import java.util.PriorityQueue;

public class Main {

	public static void main(String[] args) throws IOException {
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

		String str[] = br.readLine().split(" ");

		int n = Integer.parseInt(str[0]);
		int m = Integer.parseInt(str[1]);

		PriorityQueue<Integer> pq = new PriorityQueue<>(Comparator.reverseOrder());

		ArrayList<ArrayList<Integer>> list = new ArrayList<>();

		for (int i = 0; i <= 100001; i++) {
			list.add(new ArrayList<>());
		}

		for (int i = 0; i < n; i++) {
			str = br.readLine().split(" ");

			int a = Integer.parseInt(str[0]);
			int b = Integer.parseInt(str[1]);

			list.get(a).add(b);
		}

		long sum = 0;
		for (int i = 0; i <= m; i++) {
			for (int j = 0; j < list.get(i).size(); j++) {
				pq.add(list.get(i).get(j));
			}
			if (pq.isEmpty())
				continue;
			sum += pq.poll();
		}

		System.out.println(sum);
	}

}