Problem:
Score : 400 points

Problem Statement

There are N islands lining up from west to east, connected by N-1 bridges.

The i-th bridge connects the i-th island from the west and the (i+1)-th island from the west.

One day, disputes took place between some islands, and there were M requests from the inhabitants of the islands:

Request i: A dispute took place between the a_i-th island from the west and the b_i-th island from the west. Please make traveling between these islands with bridges impossible.

You decided to remove some bridges to meet all these M requests.

Find the minimum number of bridges that must be removed.

Constraints

All values in input are integers.

2 \leq N \leq 10^5

1 \leq M \leq 10^5

1 \leq a_i < b_i \leq N

All pairs (a_i, b_i) are distinct.

Input

Input is given from Standard Input in the following format:

N M
a_1 b_1
a_2 b_2
:
a_M b_M

Output

Print the minimum number of bridges that must be removed.

Sample Input 1

5 2
1 4
2 5

Sample Output 1

1

The requests can be met by removing the bridge connecting the second and third islands from the west.

Sample Input 2

9 5
1 8
2 7
3 5
4 6
7 9

Sample Output 2

2

Sample Input 3

5 10
1 2
1 3
1 4
1 5
2 3
2 4
2 5
3 4
3 5
4 5

Sample Output 3

4

Solution in Go (1.6) (judge verdict: Accepted)

package main

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

var sc = bufio.NewScanner(os.Stdin)

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

type AB struct {
	A int
	B int
}

type ABS []AB

func (abs ABS) Len() int {
	return len(abs)
}

func (abs ABS) Swap(i, j int) {
	abs[i], abs[j] = abs[j], abs[i]
}

func (abs ABS) Less(i, j int) bool {
	return abs[i].B < abs[j].B
}

func main() {
	sc.Split(bufio.ScanWords)

	n, m := nextInt(), nextInt()
	slice := make([]AB, m)
	for i := 0; i < m; i++ {
		slice[i] = AB{A: nextInt(), B: nextInt()}
	}
	n++

	sort.Sort(ABS(slice))

	res := 0
	end := 0 // 最後に選んだ島の位置
	for i := 0; i < m; i++ {
		if slice[i].A >= end {
			end = slice[i].B
			res++
		}
	}

	fmt.Println(res)
}