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 Scala (2.11.7) (judge verdict: Time Limit Exceeded)

object Main {

  def main(args: Array[String]): Unit = {

    val Array(n,m) = readLine().split(" ").map(_.toInt)


    val arbeiten = Array(0,0) :: (0 until n)
      .map(_ => readLine().split(" ").map(_.toInt)).toList
      
    var availableArbeiten = arbeiten.filter(_(0) <= 0).sortBy(-_(1))

    val totalReward = (for { 
      leftDays <- 0 to m
    } yield {
      val reward = availableArbeiten.headOption.map(_(1)).getOrElse(0)
      availableArbeiten = ((arbeiten.filter(_(0) == leftDays + 1)) ++ 
        (if(availableArbeiten.size == 0) Nil else availableArbeiten.tail)).sortBy(-_(1))
      reward
    }).sum

    println(totalReward)
  }

}