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 (1.15.1) (judge verdict: Accepted)

use std::io::*;

fn main() {
    let stdin = stdin();

    let mut buf = String::new();
    stdin.read_line(&mut buf).ok();
    let (n, _m) = to_pair(&buf);

    let mut conflicts: Vec<(i32, i32)> =
        stdin.lock().lines().map(|l| to_pair(&l.unwrap())).collect();
    conflicts.sort_by_key(|t| t.1);
    let mut bridges = vec![false; (n - 1) as usize];

    for t in conflicts {
        let w = (t.0 - 1) as usize;
        let e = (t.1 - 1) as usize;
        if bridges[w..e - 1].iter().all(|&b| b == false) {
            bridges[e - 1] = true;
        }
    }

    println!("{}", bridges.into_iter().filter(|&b| b).count());
}

fn to_pair(s: &str) -> (i32, i32) {
    let v: Vec<_> = s.split_whitespace().map(|w| w.parse().unwrap()).collect();
    let (l, r) = (v[0], v[1]);
    (l, r)
}