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.*;

public class Main {
    public static void main(String[] args) throws Exception {
        // Your code here!
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int m = sc.nextInt();
        
        // list[i]: i日後に報酬がもらえる各仕事の報酬
        List<Integer>[] list = new List[100101];
        for (int i = 0; i < 100101; i++) {
            list[i] = new ArrayList<Integer>();
        }
        
        // a日後にb円もらえる仕事をリストに入れる
        for (int i = 0; i < n; i++) {
            int a = sc.nextInt();
            int b = sc.nextInt();
            list[a].add(b);
        }
        
        // que: 今できる仕事のキュー(報酬の大きい順に並んでいる)
        PriorityQueue<Integer> que = new PriorityQueue<>(Collections.reverseOrder());
        
        long ans = 0;
        for (int i = 1; i <= m; i++) {
            // 残りi日でできる仕事の報酬bをキューに入れる
            for (Integer b : list[i]) {
                que.add(b);
            }
            
            // 今できる仕事のうち、最大の報酬の仕事を選ぶ
            if (!que.isEmpty()) {
                ans += que.poll();
            }
        }
        
        System.out.println(ans);
    }
}