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.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.Map;
import java.util.Scanner;
import java.util.TreeMap;

public class Main {

    static Map<Integer, List<Integer>> baMap = new TreeMap<>(Collections.reverseOrder());

    /**
     * @param args
     */
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int n = 0;
        int m = 0;
        int i = 0;

        while (true) {
            String inputText = scanner.nextLine();
            String[] inputTextArr = inputText.split(" ");

            if (i == 0) {
                n = Integer.parseInt(inputTextArr[0]);
                m = Integer.parseInt(inputTextArr[1]);
            } else {
                Integer a = Integer.valueOf(inputTextArr[0]);
                Integer b = Integer.valueOf(inputTextArr[1]);

                List<Integer> aList = baMap.get(b);
                if (aList == null) {
                    aList = new ArrayList<>();
                }
                aList.add(a);
                baMap.put(b, aList);
            }

            if (i >= n) {
                break;
            }

            i++;

        }
        scanner.close();

        int result = 0;
        int day = 1;

        for (; day <= m; day++) {
            result = result + getPay(day);
        }

        System.out.println(result);
    }

    private static Integer getPay(Integer day) {
        for (Map.Entry<Integer, List<Integer>> entry : baMap.entrySet()) {
            List<Integer> aList = entry.getValue();
            Collections.sort(aList);
            for (int idx = 0; idx < aList.size(); idx++) {
                Integer a = aList.get(idx);
                if (a <= day) {
                    aList.remove(idx);
                    baMap.put(entry.getKey(), aList);
                    return entry.getKey();
                }
            }
        }
        return 0;
    }

}