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.io.IOException;
import java.io.InputStream;
import java.util.Comparator;
import java.util.NoSuchElementException;
import java.util.PriorityQueue;

public class Main {
	public static void main(String[] args) {
		FastScanner2 fs = new FastScanner2();

		int N = fs.nextInt();
		int M = fs.nextInt();

		PriorityQueue<Baito>[] pqList= new PriorityQueue[M + 1];
		for(int i = 1;i <= M;i++) {
			pqList[i] = new PriorityQueue<Baito>(new MyComparator2());
		}

		for(int i = 1;i <= N;i++) {
			int A = fs.nextInt();
			int B = fs.nextInt();
			Baito baito = new Baito(A, B);
			if(A <= M) pqList[A].add(baito);
		}

		PriorityQueue<Baito> maxPq = new PriorityQueue<Baito>(new MyComparator2());
		int sum = 0;
		for(int i = 1;i <= M;i++) {
//			System.out.println("i:" + i);
			Baito baito = pqList[i].poll();
			if(baito != null) {
				maxPq.add(baito);
			}
			baito = maxPq.poll();
			if(baito != null) {
				Baito baito2 = pqList[baito.A].poll();
				if(baito2 != null) maxPq.add(baito2);
				sum += baito.B;
			}
		}
		System.out.println(sum);

	}
}

class MyComparator2 implements Comparator<Baito>{
	@Override
	public int compare(Baito b1, Baito b2) {
		if(b1.B > b2.B) {
			return -1;
		}else if(b1.B < b2.B) {
			return 1;
		}else {
			return 0;
		}
	}
}

class Baito{
	public int A;
	public int B;
	public Baito(int a, int b) {
		A = a;
		B = b;
	}

}

class FastScanner2 {
    private final InputStream in = System.in;
    private final byte[] buffer = new byte[1024];
    private int ptr = 0;
    private int buflen = 0;
    private boolean hasNextByte() {
        if (ptr < buflen) {
            return true;
        }else{
            ptr = 0;
            try {
                buflen = in.read(buffer);
            } catch (IOException e) {
                e.printStackTrace();
            }
            if (buflen <= 0) {
                return false;
            }
        }
        return true;
    }
    private int readByte() { if (hasNextByte()) return buffer[ptr++]; else return -1;}
    private static boolean isPrintableChar(int c) { return 33 <= c && c <= 126;}
    public boolean hasNext() { while(hasNextByte() && !isPrintableChar(buffer[ptr])) ptr++; return hasNextByte();}
    public String next() {
        if (!hasNext()) throw new NoSuchElementException();
        StringBuilder sb = new StringBuilder();
        int b = readByte();
        while(isPrintableChar(b)) {
            sb.appendCodePoint(b);
            b = readByte();
        }
        return sb.toString();
    }
    public long nextLong() {
        if (!hasNext()) throw new NoSuchElementException();
        long n = 0;
        boolean minus = false;
        int b = readByte();
        if (b == '-') {
            minus = true;
            b = readByte();
        }
        if (b < '0' || '9' < b) {
            throw new NumberFormatException();
        }
        while(true){
            if ('0' <= b && b <= '9') {
                n *= 10;
                n += b - '0';
            }else if(b == -1 || !isPrintableChar(b)){
                return minus ? -n : n;
            }else{
                throw new NumberFormatException();
            }
            b = readByte();
        }
    }
    public int nextInt() {
        long nl = nextLong();
        if (nl < Integer.MIN_VALUE || nl > Integer.MAX_VALUE) throw new NumberFormatException();
        return (int) nl;
    }
    public double nextDouble() { return Double.parseDouble(next());}
}