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: Wrong Answer)

package main

import (
	"bufio"
	"fmt"
	"os"
	"sort"
	"strconv"
)

func newScanner() *bufio.Scanner {
	scanner := bufio.NewScanner(os.Stdin)
	scanner.Split(bufio.ScanWords)
	return scanner
}

var sc = newScanner()

func scanInt() int {
	sc.Scan()
	v, _ := strconv.Atoi(sc.Text())
	return v
}

func scanInts(n int) []int {
	a := make([]int, n)
	for i := 0; i < n; i++ {
		a[i] = scanInt()
	}
	return a
}

func scanString() string {
	if sc.Scan() {
		return sc.Text()
	}
	panic(sc.Err())
}

type Task struct {
	a int
	b int
}

type sortTasks []Task

func (s sortTasks) Less(i, j int) bool {
	if s[i].b == s[j].b {
		return s[i].a > s[j].a
	}
	return s[i].b > s[j].b
}

func (s sortTasks) Swap(i, j int) {
	s[i], s[j] = s[j], s[i]
}

func (s sortTasks) Len() int {
	return len(s)
}

func main() {
	n, m := scanInt(), scanInt()
	tasks := make([]Task, n)
	for i := 0; i < n; i++ {
		tasks[i] = Task{scanInt(), scanInt()}
	}
	answer := maxRevenue(n, m, tasks)
	fmt.Println(answer)
}

func debug(a ...interface{}) {
	// fmt.Println(a...)
}

func maxRevenue(n, m int, tasks []Task) int {
	sort.Sort(sortTasks(tasks))
	revenue := 0
	index := 0
	debug(tasks)
	for i := m; i > 0; i-- {
		debug("i:", i)
		for ; index < n; index++ {
			if tasks[index].a <= i {
				revenue += tasks[index].b
				index++
				break
			}
		}
	}
	return revenue
}