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.Scanner;
import java.util.HashMap;
import java.util.Map;
import java.io.PrintWriter;
import java.util.Collections;
import java.util.List;
import java.util.ArrayList;
import java.util.Comparator;

public class Main { //クラス名はMain
    public static void main(String[] args) {
      Scanner sc = new Scanner(System.in);
      PrintWriter out = new PrintWriter(System.out);
      String[] paras = sc.next().split(" ");
      int n = Integer.parseInt(paras[0]);
      int m = Integer.parseInt(paras[1]);
      Map<Integer,List<Integer>> map = new HashMap<>();
      for(int i = 0;i<n;i++){
        String[] reward = sc.next().split(" ");
        int day = Integer.parseInt(reward[0]);
        int num = Integer.parseInt(reward[1]);
        if(day > m) continue;
        if(map.containsKey(day)){
          List<Integer> l = map.get(day);
          l.add(num);
          map.put(day,l);
        }else{
          List<Integer> l = new ArrayList<>();
          l.add(num);
          map.put(day,l);
        }
      }
      
      int[] taken = new int[m+1];
      for(int i = m;i>=1;i--){
        if(map.containsKey(i)){
          List<Integer> l = map.get(i);
          Collections.sort(l,Comparator.reverseOrder());
          map.put(i,l);
        }else taken[i] = -1;
      }
      
      long ans = 0;
      for(int i = m;i>=1;i--){
        int max = 0;
        int last = 0;
        for(int j = i;j<=m;j++){
          if(taken[j] < 0) continue;
          List<Integer> l = map.get(j);
          if(taken[j] >=l.size()) continue;
          if(l.get(taken[j]) > max){
            max = l.get(taken[j]);
            taken[last]--;
            taken[j]++;
            last = j;
          }
        }
        ans += max;
      }
      
      out.println(ans);
      out.flush();
    }
}