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 PHP7 (7.0.15) (judge verdict: Accepted)

<?php

while (true) {
	$input = trim(fgets(STDIN));
	$input = explode(" ", $input);
	foreach ($input as $value) {
		$args[] = $value;
		if (count($args) >= 2) {
			break;
		}
	}
	if (count($args) >= 2) {
		break;
	}
}


$islands = (int)$args[0];
$request_cnt = (int)$args[1];
$b_count = $islands-1;
$b_array = [];
$cnt = 0;
$requests=[];

while ($cnt < $request_cnt) {
	$input = trim(fgets(STDIN));
	$input = explode(" ", $input);
	$requests[] =[
		"from" => (int)$input[0],
		"to" => (int)$input[1]
	];
	++$cnt;
}
usort($requests, "my_sort");

$result = 0;
$prev = -1;
foreach ($requests as $request) {
	$from = $request["from"];
	$to = $request["to"];
	if ((int)$from < $prev) {
		continue;
	}
	++$result;
	$prev = $to;
}


out($result);



function out($var){
	echo $var , PHP_EOL;
}

function my_sort($a, $b){
	if ($a["to"] == $b["to"]) {
		return $a["from"] < $b["from"] ? -1 : 1;
	}else{
		return $a["to"] < $b["to"] ? -1 : 1;
	}
}