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){
		try(Scanner sc = new Scanner(System.in)) {

			int n = sc.nextInt();
			int m = sc.nextInt();

			int[] aArray = new int[n];
			int[] bArray = new int[n];
			Pair[] pArray = new Pair[n];

			for(int i = 0 ; i < n ; i++ ) {
				aArray[i] = sc.nextInt();
				bArray[i] = sc.nextInt();
				Pair p = new Pair();
				p.a = aArray[i];
				p.b = bArray[i];
				pArray[i] = p;
			}

			//
//			Arrays.stream(pArray).forEach(p -> System.out.println("## (" + p.a  + "," + p.b + ")"));

			//backward
			//rev_m = 0 to M-1 find max(a) where b <= rev_m
			//1,2,3,4,5
			//5,4,3,2,1

			Arrays.sort(pArray, new Comparator<Pair>() {

				@Override
				public int compare(Pair o1, Pair o2) {
					return Integer.compare(o1.a, o2.a);
				}
			});

//			Arrays.stream(pArray).forEach(p -> System.out.println("## (" + p.a  + "," + p.b + ")"));

			PriorityQueue<Pair> pq = new PriorityQueue<>(
					new Comparator<Pair>() {
						@Override
						public int compare(Pair o1, Pair o2) {
							return Integer.compare(o2.b, o1.b);
						}
					});
			
			int po = 0;
			long sum = 0;
			for(int revm = 1 ; revm <= m ; revm++ ) {
				while(po < pArray.length && pArray[po].a <= revm ) {
//					System.out.println("add " + pArray[po].a + "," + pArray[po].b);
					pq.add(pArray[po]);
					po++;
				}
				//get largest
				Pair max = pq.poll();
				if ( max != null ) {
					sum += max.b;
				}
			}
			
			System.out.println(sum);

		}
	}

	public static class Pair {
		public int a;
		public int b;
	}
}