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 io::*;
use std::cmp::min;

fn main() {
	let con = read_string();
	let mut sc = Scanner::new(&con);

	let n: i32 = sc.next();
	let m: i32 = sc.next();
	
	let mut v: Vec<(i32, i32)> = (0..m).map(|_| (sc.next(), sc.next())).collect();

	v.sort();
	let mut ans = 1;
	let mut b = n;
	for i in 0..m as usize {
		if v[i].0 < b {
			b = min(b, v[i].1);
		} else {
			b = v[i].1;
			ans += 1;
		}
	}
	
	println!("{}", ans);
}

pub mod io {
    use std;
    use std::str::FromStr;
 
    pub struct Scanner<'a> {
        iter: std::str::SplitWhitespace<'a>,
    }
 
    impl<'a> Scanner<'a> {
        pub fn new(s: &'a str) -> Scanner<'a> {
            Scanner {
                iter: s.split_whitespace(),
            }
        }
 
        pub fn next<T: FromStr>(&mut self) -> T {
            let s = self.iter.next().unwrap();
            if let Ok(v) = s.parse::<T>() {
                v
            } else {
                panic!("Parse error")
            }
        }
 
        pub fn next_vec_len<T: FromStr>(&mut self) -> Vec<T> {
            let n: usize = self.next();
            self.next_vec(n)
        }
 
        pub fn next_vec<T: FromStr>(&mut self, n: usize) -> Vec<T> {
            (0..n).map(|_| self.next()).collect()
        }
    }
 
    pub fn read_string() -> String {
        use std::io::Read;
 
        let mut s = String::new();
        std::io::stdin().read_to_string(&mut s).unwrap();
        s
    }
 
    pub fn read_line() -> String {
        let mut s = String::new();
        std::io::stdin().read_line(&mut s).unwrap();
        s.trim_right().to_owned()
    }
}