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 (
    "fmt"
    "sort"
)

type As [][]int

func (a As) Len() int {
    return len(a)
}

func (a As) Swap(i, j int) {
    a[i], a[j] = a[j], a[i]
}

func (a As) Less(i, j int) bool {
    if a[i][1] == a[j][1] {
        return a[i][0] < a[j][0]
    }
    return a[i][1] < a[j][1]
}

func main() {
    var n, m int
    fmt.Scan(&n, &m)
    as := make(As, m)
    for i := 0; i < m; i++ {
        var a, b int
        fmt.Scan(&a, &b)
        b--
        as[i] = []int{a, b}
    }
    sort.Sort(as)
    var sum int
    var lb int
    for i := 0; i < m; i++ {
        a, b := as[i][0], as[i][1]
        if a > lb {
            lb = b
            sum++
        }
    }
    fmt.Println(sum)
}