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::stdin;
fn get_string() -> String {
    let mut s = String::new();
    stdin().read_line(&mut s).unwrap();
    s.trim().to_string()
}

macro_rules! input {
    ($single:ty) => ( get_string().parse::<$single>().unwrap() );
    ($plural:ty,) => ({
        let line = get_string();
        line.split_whitespace().map(|v| v.parse::<$plural>().unwrap()).collect::<Vec<$plural>>()
    });
    ($($plural:ty),+) => ({
        let line = get_string();
        let mut iter = line.split_whitespace();
        ($(iter.next().unwrap().parse::<$plural>().unwrap()),+)
    });
    ($single:ty; $times:expr) => ( (0..$times).map(|_| input!($single)).collect::<Vec<$single>>() );
    ($($plural:ty),+; $times:expr) => ( (0..$times).map(|_| input!($($plural),+)).collect::<Vec<($($plural),+)>>() );
}

#[allow(dead_code)]
fn get_value<T: std::str::FromStr>() -> T {
    get_string().parse().ok().unwrap()
}
#[allow(dead_code)]
fn get_values_for<T: std::str::FromStr>(times: u64) -> Vec<T> {
    (0..times).map(|_| get_value()).collect()
}
#[allow(dead_code)]
fn get_vector<T: std::str::FromStr>() -> Vec<T> {
    get_string()
        .split_whitespace()
        .map(|v| v.parse().ok().unwrap())
        .collect()
}
#[allow(dead_code)]
fn get_vectors_for<T: std::str::FromStr>(times: u64) -> Vec<Vec<T>> {
    (0..times).map(|_| get_vector()).collect()
}

fn main() {
    let (_n, m) = input!(u64, u64);
    let mut req = input!(u64, u64; m);
    req.sort_by_key(|v| v.1);
    let mut ans = 0;
    let mut bb = 0;
    for r in req {
        if r.0 <= bb {
            continue;
        }
        ans += 1;
        bb = r.1 - 1;
    }
    println!("{}", ans);
}