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: Wrong Answer)

<?php
class node
{
    public $a;
    public $b;
    public function __construct($var1,$var2)
    {
        $this->a = $var1;
        $this->b = $var2;
    }
}
$arr = array();
fscanf(STDIN,"%d %d",$N,$M);
for($i = 0;$i < $M;$i++)
{
    fscanf(STDIN,"%d %d",$a,$b);
    if(!isset($arr[$a]))
       $arr[$a] = $b;
    else{
        $arr[$a] = min($arr[$a],$b);
    }
}
$array = array();
for($i = 1;$i <= $N;$i++)
{
    if(isset($arr[$i]))
    {
      array_push($array,new node($i,$arr[$i]));
    }
}
function cmp($var1,$var2)
{
    return $var1->b > $var2->b;
}
usort($array,"cmp");
$len = count($array);
/*for($i = 0;$i < $len ;$i++)
{
    echo $array[$i]->a;
    echo " ";
    echo $array[$i]->b;
    echo"\n";
}*/
if($len === 0)
{
    echo "0\n";
}
else 
{
    $cnt = 1;
    $m_right = $array[0]->b-1;
    for($i = 1;$i < $len;$i++)
    {
        if($array[$i]->a <= $m_right)
        {
            continue;
        }
        $cnt++;
        $m_right = $array[$i]->b;
    }
    echo $cnt."\n";
}