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: Time Limit Exceeded)

import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.List;
import java.util.Scanner;
import java.util.Comparator;
import java.util.ArrayList;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 *
 * @author teiwa
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        Scanner in = new Scanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        TaskD solver = new TaskD();
        solver.solve(1, in, out);
        out.close();
    }

    static class TaskD {
        public void solve(int testNumber, Scanner in, PrintWriter out) {

            int N = in.nextInt();
            int M = in.nextInt();
            ArrayList<Integer>[] l = new ArrayList[M];
            for (int i = 0; i < M; i++) {
                l[i] = new ArrayList<Integer>();
            }
            for (int i = 0; i < N; i++) {
                int A = in.nextInt();
                int B = in.nextInt();
                if (M - A >= 0) {
                    l[M - A].add(B);
                }
            }
            int ans = 0;
            //PriorityQueue<Integer> q = new PriorityQueue<Integer>(Comparator.reverseOrder());
            List<Integer> q = new ArrayList<>();
            for (int i = M - 1; i >= 0; i--) {
                for (int b : l[i]) {
                    q.add(b);
                }
                if (!q.isEmpty()) {
                    //ans += q.poll();
                    q.sort(Comparator.reverseOrder());
                    ans += q.get(0);
                    q.remove(0);
                }
            }
            out.println(ans);

        }

    }
}