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.Arrays;
import java.util.Comparator;
import java.util.PriorityQueue;
import java.util.Scanner;

public class Main {

  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    int M = sc.nextInt();
    AB[] ab = new AB[N];
    PriorityQueue<Integer> queue = new PriorityQueue<>(Comparator.reverseOrder());
    for (int i = 0; i < N; i++) {
      ab[i] = new AB(sc.nextInt(), sc.nextInt());
    }
    Arrays.sort(ab, (a, b) -> a.a - b.a);
    int ans = 0;
    int ptr = 0;
    for (int i = 1; i <= M; i++) {
      while (ptr < ab.length && ab[ptr].a <= i) {
        queue.add(ab[ptr].b);
        ptr++;
      }
      if (queue.isEmpty()) continue;
      ans += queue.poll();
    }

    System.out.println(ans);
  }

  static class AB {
    int a, b;

    AB(int a, int b) {
      this.a = a;
      this.b = b;
    }
  }
}