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: Runtime Error)

import java.util.*;
import java.util.Map.Entry;

public class Main {
    public static void main(String[] args) throws Exception {
        // Your code here!
        Scanner scan = new Scanner(System.in);
        Integer N = scan.nextInt();
        Integer M = scan.nextInt();
        long totalEarn = 0;
        Map<Integer, List<Integer>> map = new TreeMap<Integer, List<Integer>>();
        for(int i = 0; i < N; i++){
            Integer A = scan.nextInt();
            Integer B = scan.nextInt();
            if(A > M)
                continue;

            if(map.containsKey(A)){
                List<Integer> list = map.get(A);
                list.add(B);
                Collections.sort(list, Comparator.reverseOrder());
            } else {    
                List<Integer> list = new ArrayList<Integer>();
                list.add(B);
                map.put(A, list);
            }
        }
        if(map.size() > 0) {
          List<Integer> candidate = new ArrayList<Integer>();
          for(int i = 1; i < M + 1; i++) {
              if(map.containsKey(i)) {
                  if(map.get(i).isEmpty())
                      continue;

                  for(Integer earn : map.get(i)) {
                      candidate.add(earn);
                  }
                  Collections.sort(candidate, Comparator.reverseOrder());
              }   
              System.out.println(candidate);
              totalEarn += candidate.get(0);
              candidate.remove(0);
          }
        }
        System.out.println(totalEarn);
    }
}