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 Rust (1.42.0) (judge verdict: Accepted)

use proconio::{fastout, input, marker::Usize1};
use std::cmp::Ordering;

#[fastout]
fn main() {
    input! {
        n: usize,
        m: usize,
        mut ab: [(Usize1, Usize1); m],
    }
    ab.sort_by(|x, y| {
        if x.1 < y.1 {
            Ordering::Less
        } else if x.1 > y.1 || x.0 > y.0 {
            Ordering::Greater
        } else if x.0 < y.0 {
            Ordering::Less
        } else {
            Ordering::Equal
        }
    });

    let mut res = 1;
    let mut now = ab[0].1;
    for i in 1..ab.len() {
        if now <= ab[i].0 {
            res += 1;
            now = ab[i].1;
        }
    }

    println!("{}", res);
}