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"
	"log"
	"math"
	"os"
	"sort"
	"strconv"
)

var sc = bufio.NewScanner(os.Stdin)
var buff []byte

func init() {
	sc.Split(bufio.ScanWords)
	sc.Buffer(buff, bufio.MaxScanTokenSize*1024)
	log.SetFlags(log.Lshortfile)
}

func main() {
	N := nextInt()
	M := nextInt()
	reward := make([]Pair, 0)
	for i := 0; i < N; i++ {
		var p Pair
		p.a = nextInt()
		p.b = nextInt()
		reward = append(reward, p)
	}
	sort.Sort(sort.Reverse(Rewards(reward)))
	log.Println(reward)
	ans := 0
	for i := 1; i <= M; i++ {
		for j := 0; j < len(reward); j++ {
			if reward[j].a <= i {
				ans += reward[j].b
				reward = append(reward[:j], reward[j+1:]...)
				break
			}
		}
	}
	fmt.Println(ans)
}

type Rewards []Pair

func (r Rewards) Len() int {
	return len(r)
}

func (r Rewards) Less(i, j int) bool {
	return r[i].b < r[j].b
}

func (r Rewards) Swap(i, j int) {
	r[i], r[j] = r[j], r[i]
}

type Days []Pair

func (d Days) Len() int {
	return len(d)
}

func (d Days) Less(i, j int) bool {
	return d[i].a < d[j].a
}

func (d Days) Swap(i, j int) {
	d[i], d[j] = d[j], d[i]
}

func nextString() string {
	sc.Scan()
	return sc.Text()
}

func nextFloat64() float64 {
	sc.Scan()
	f, err := strconv.ParseFloat(sc.Text(), 64)
	if err != nil {
		panic(err)
	}
	return f
}

func nextInt() int {
	sc.Scan()
	i, err := strconv.Atoi(sc.Text())
	if err != nil {
		panic(err)
	}
	return i
}

func nextInts(n int) (r []int) {
	r = make([]int, n)
	for i := 0; i < n; i++ {
		r[i] = nextInt()
	}
	return r
}

var dy = []int{0, 1, 0, -1}
var dx = []int{1, 0, -1, 0}
var MAX = math.MaxInt32

func pow(a, b int) int {
	return int(math.Pow(float64(a), float64(b)))
}

func lcm(a, b int) int {
	return a * b / gcd(a, b)
}
func gcd(a, b int) int {
	if a < b {
		a, b = b, a
	}
	for b != 0 {
		a, b = b, a%b
	}
	return a
}
func maxInt(a ...int) int {
	r := a[0]
	for i := 0; i < len(a); i++ {
		if r < a[i] {
			r = a[i]
		}
	}
	return r
}
func minInt(a ...int) int {
	r := a[0]
	for i := 0; i < len(a); i++ {
		if r > a[i] {
			r = a[i]
		}
	}
	return r
}
func sum(a []int) (r int) {
	for i := range a {
		r += a[i]
	}
	return r
}
func abs(a int) int {
	if a < 0 {
		return -a
	}
	return a
}

type Pair struct {
	a, b int
}

type Pairs []Pair

func (p Pairs) Len() int {
	return len(p)
}
func (p Pairs) Swap(i, j int) {
	p[i], p[j] = p[j], p[i]
}

func (p Pairs) Less(i, j int) bool {
	if p[i].a == p[j].a {
		return p[i].b < p[j].b
	}
	return p[i].a < p[j].a
}