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;

pub fn main(){
    let mut instr = String::new();
    io::stdin().read_line(&mut instr).expect("!");
    let instr = instr.trim().split(' ').map(|ref x|x.parse::<i32>().unwrap()).collect::<Vec<i32>>();
    let (N, M) = (instr[0], instr[1]);
    
    let mut paths = Vec::<(i32, i32)>::new();
    for _ in 0..M {
        let mut instr = String::new();
        io::stdin().read_line(&mut instr).expect("!");
        let instr = instr.trim().split(' ').map(|ref x|x.parse::<i32>().unwrap()).collect::<Vec<i32>>();
        
        let (l, r) = (instr[0], instr[1]);
        paths.push((l, r));
    }

    paths.sort_by_key(|ref k| k.1);

    let mut prev_r = -1;
    let mut result = 0;
    for (l, r) in paths{
        let r = r - 1;
        if prev_r < l {
            prev_r = r;
            result += 1;
        }
    }
    println!("{}", result);

}