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::cmp::min;
 
fn main() {
    let nm: Vec<i32> = read_vec();
    let (n, m) = (nm[0], nm[1]);

    let mut v: Vec<(i32, i32)> = (0..m).map(|_| {
        let ab: Vec<i32> = read_vec();
        (ab[0], ab[1])
        }).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);
}
 
fn read_vec<T: std::str::FromStr>() -> Vec<T> {
    let mut buf = String::new();
    std::io::stdin().read_line(&mut buf).unwrap();
    buf.trim().split_whitespace().map(|e|
        e.parse().ok().unwrap()
    ).collect()
}