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 (
	"bufio"
	"fmt"
	"io"
	"os"
	"sort"
	"strconv"
	"strings"

)

func main() {
	solve(os.Stdin, os.Stdout)
}

type ReaderEx struct {
	baseScanner *bufio.Scanner
	words       []string
	nextWordIdx func() int
	initWordIdx func()
}

func NewScannerEx(stdin io.Reader) *ReaderEx {
	var this ReaderEx
	var idx int
	this.baseScanner = bufio.NewScanner(stdin)
	this.nextWordIdx = func() int {
		cur := idx
		idx++
		return cur
	}
	this.initWordIdx = func() {
		idx = 0
	}
	return &this
}

func (me *ReaderEx) ScanAndSplit() (isScanned bool) {
	isScanned = me.baseScanner.Scan()

	me.words = strings.Fields(me.baseScanner.Text())
	me.initWordIdx()
	return
}

func (me *ReaderEx) getString() string {
	return me.baseScanner.Text()
}

func (me *ReaderEx) getInt() int {
	n, err := strconv.Atoi(me.baseScanner.Text())
	defer func() {
		if err != nil {
			fmt.Fprintln(os.Stderr, "getInt()")
		}
	}()

	return n
}

func (me *ReaderEx) nextBytes() []byte {
	return []byte(me.words[me.nextWordIdx()])
}
func (me *ReaderEx) nextString() string {
	return me.words[me.nextWordIdx()]
}
func (me *ReaderEx) nextInt() int {
	n, err := strconv.Atoi(me.words[me.nextWordIdx()])

	defer func() {
		if err != nil {
			fmt.Fprintln(os.Stderr, "getInt(", n, ")")
		}
	}()
	return n
}

type inInfo struct {
	in    int
	after int
}

type inInfoSorter []inInfo

func (a inInfoSorter) Len() int      { return len(a) }
func (a inInfoSorter) Swap(i, j int) { a[i], a[j] = a[j], a[i] }
func (a inInfoSorter) Less(i, j int) bool {
	switch {
	case a[i].in > a[j].in:
		return true
	case a[i].in < a[j].in:
		return false
	case a[i].after > a[j].after:
		return true
	default:
		return false
	}
}

func sum(w []int) int {
	s := 0
	for _, v := range w {
		s += v
	}
	return s
}

type ptr []int

func (x ptr) top(i int) int {
	ptrs := []int(x)
	var his []int
	cur := i
	for ; cur >= 0 && ptrs[cur] != cur; cur = ptrs[cur] {
		his = append(his, cur)
	}
	for _, v := range his {
		ptrs[v] = cur - 1
	}
	if cur >= 0 {
		ptrs[cur] = cur - 1
	}
	return cur
}

func solve(stdin io.Reader, stdout io.Writer) {
	var n, m int
	var infos []inInfo
	reader := NewScannerEx(stdin)
	reader.ScanAndSplit()
	n, m = reader.nextInt(), reader.nextInt()

	for i := 0; i < n; i++ {
		reader.ScanAndSplit()
		infos = append(infos, inInfo{after: reader.nextInt(), in: reader.nextInt()})
	}

	sort.Sort(inInfoSorter(infos))

	var ans = make([]int, m)

	for _, w := range infos {
		i := m - w.after
		for ; i >= 0 && ans[i] > 0; i-- {

		}
		if i >= 0 {
			ans[i] = w.in
		}
	}

	fmt.Fprintln(stdout, sum(ans))

}