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: Time Limit Exceeded)

use std::io;
use std::str::FromStr;

type Range = (u64, u64);

fn input_int_tuple() -> (u64, u64) {
    let mut line: String = String::new();
    io::stdin().read_line(&mut line);
    let vec: Vec<u64> = line.split_whitespace().map(|s| s.parse().unwrap()).collect();
    (vec[0], vec[1])
}

fn main() {
    let (n, m) = input_int_tuple();
    let mut range_vec: Vec<Range> = Vec::new();
    for i in 0..m {
        let range = input_int_tuple();
        range_vec.push(range);
    }
    range_vec.sort_by(|a, b| a.1.cmp(&b.1));
    let mut satisfied: Vec<usize> = Vec::new();
    let mut count = 0;
    for i in 0..range_vec.len() as usize {
        if satisfied.contains(&i) {
            continue;
        }
        let cut = range_vec[i].1 - 1;
        count += 1;
        for j in 0..range_vec.len() {
            if range_vec[j].0 <= cut {
                satisfied.push(j);
            }
        }        
    }
    // println!("{:?}", range_vec);
    println!("{}", count);
}