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 Kotlin (1.0.0) (judge verdict: Accepted)

fun main(args: Array<String>) {
    val (n, m) = readLine()!!.split(" ").map(String::toInt)
    val xs: MutableList<Pair<Int, Int>> = mutableListOf()
    for (i in 1..m) {
        val (a, b) = readLine()!!.split(" ").map(String::toInt)
        xs.add(Pair(a, b))
    }

    xs.sortBy { it.second }

    var count = 0
    var prev = 0
    if (xs.isNotEmpty()) {
        count = 1
        prev = xs[0].second - 1
    }

    for (i in 1 until xs.size) {
        val pair = xs[i]
        if (pair.first <= prev) {
            continue
        }

        count++
        prev = pair.second - 1
    }

    println(count)
}