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: Accepted)

package main

import (
	"bufio"
	"container/heap"
	"fmt"
	"os"
	"strconv"
)

type priorityQueue []int

func (p priorityQueue) Len() int            { return len(p) }
func (p priorityQueue) Less(i, j int) bool  { return p[i] > p[j] }
func (p priorityQueue) Swap(i, j int)       { p[i], p[j] = p[j], p[i] }
func (p *priorityQueue) Push(x interface{}) { *p = append(*p, x.(int)) }
func (p *priorityQueue) Pop() interface{} {
	old := *p
	n := len(old)
	x := old[n-1]
	*p = old[0 : n-1]
	return x
}

func main() {
	var n, m int
	fmt.Scan(&n, &m)

	sc := bufio.NewScanner(os.Stdin)
	sc.Split(bufio.ScanWords)
	ts := make(map[int][]int)
	for i := 0; i < n; i++ {
		sc.Scan()
		a, _ := strconv.Atoi(sc.Text())
		sc.Scan()
		b, _ := strconv.Atoi(sc.Text())
		ts[a] = append(ts[a], b)
	}

	pq := make(priorityQueue, 0, n)
	heap.Init(&pq)
	ans := 0
	for i := 1; i <= m; i++ {
		for _, v := range ts[i] {
			heap.Push(&pq, v)
		}
		if len(pq) > 0 {
			ans += heap.Pop(&pq).(int)
		}
	}
	fmt.Println(ans)
}