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
// D
fscanf(STDIN, '%d %d', $n, $m);
for ($i  = 0; $i < $m; $i++) {
    fscanf(STDIN, '%d %d', $a, $b);
    $a--;
    $b--;
    if (isset($x[$a])) {
        $x[$a] = min($x[$a], $b);
    } else {
        $x[$a] = $b;
    }
}
sort($x);
$ans = 0;
$prem = -1; // 直前に取り除いた橋
foreach ($x as $l => $r) {
    if ($l > $prem) {
        $ans++;
        $prem = $r - 1;
    }
}
echo $ans;