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)

#![allow(non_snake_case)]

use std::io::{stdin, Read};
use std::str::FromStr;
fn read_option<T: FromStr>() -> Option<T> {
    let stdin = stdin();
    let stdin = stdin.lock();
    let token: String = stdin
        .bytes()
        .map(|c| c.expect("failed to read char") as char)
        .skip_while(|c| c.is_whitespace())
        .take_while(|c| !c.is_whitespace())
        .collect();
    token.parse().ok()
}
fn read<T: FromStr>() -> T {
    let opt = read_option();
    opt.expect("failed to parse token")
}

#[derive(Clone)]
enum Edge {
    Start(usize),
    End(usize),
}

fn main() {
    let N: usize = read();
    let M: usize = read();
    let mut vec: Vec<Vec<Edge>> = vec![vec![]; N];
    for i in 0..M {
        let a = read::<usize>() - 1;
        let b = read::<usize>() - 1;
        vec[a].push(Edge::Start(i));
        // 0以下の場合は
        vec[b].push(Edge::End(i));
    }

    use std::collections::HashSet;
    let mut set = HashSet::new();

    let mut ans = 0;
    for actions in vec {
        let mut starts = vec![];
        let mut ends = vec![];
        for v in actions {
            match v {
                Edge::Start(v) => starts.push(v),
                Edge::End(v) => ends.push(v),
            }
        }
        if ends.iter().any(|v| set.contains(v)) {
            set.clear();
            ans += 1;
        }
        for v in starts {
            set.insert(v);
        }
    }
    println!("{}", ans);
}