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 Go (1.6) (judge verdict: Time Limit Exceeded)

package main

import (
	"fmt"
	"sort"
)

// Job is ...
type Job struct {
	A int
	B int
}

// AscABy is ...
type AscABy []Job

func (a AscABy) Len() int      { return len(a) }
func (a AscABy) Swap(i, j int) { a[i], a[j] = a[j], a[i] }
func (a AscABy) Less(i, j int) bool {
	return a[i].A < a[j].A
}

// DescBy is ...
type DescBy []int

func (a DescBy) Len() int      { return len(a) }
func (a DescBy) Swap(i, j int) { a[i], a[j] = a[j], a[i] }
func (a DescBy) Less(i, j int) bool {
	return a[j] < a[i]
}

// main is ...
func main() {
	var n, m, a, b int

	fmt.Scanln(&n, &m)

	jobList := make([]Job, n)
	for i := 0; i < n; i++ {
		fmt.Scanln(&a, &b)
		if m < a {
			continue
		}
		jobList[i] = Job{a, b}
	}

	// A Asc JobList sort
	sort.Sort(AscABy(jobList))

	retList := []int{}
	ret, next := 0, 0
	size := len(jobList)
	if m < len(jobList) {
		size = m
	}

	for i := 1; i <= m; i++ {
		for ; next < len(jobList); next++ {
			if jobList[next].A > i {
				break
			}
			retList = append(retList, jobList[next].B)
			continue
		}

		if 0 == len(retList) {
			continue
		}
		sort.Sort(DescBy(retList))
		size = m - i + 1
		if len(retList) > size {
			retList = retList[:size]
		}

		ret += retList[0]
		retList[0] = 0
	}
	fmt.Print(ret)
}