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 Java (OpenJDK 11.0.6) (judge verdict: Accepted)


import java.io.PrintWriter;
import java.util.*;

public class Main {
    void run() {
        Scanner sc = new Scanner(System.in);
        PrintWriter out = new PrintWriter(System.out);

        int n = sc.nextInt();
        int m = sc.nextInt();

        int[][] reqs = new int[m][2];

        for (int i = 0; i < m; i++) {
            reqs[i] = new int[]{sc.nextInt(), sc.nextInt()};
        }

        Arrays.sort(reqs, (a1, a2) -> a1[1] - a2[1]);
//        debug(reqs);

        int count = 0;
        int max = 0;
        for (int i = 0; i < m; i++) {
            int[] req = reqs[i];
            if (max <= req[0]) {
                max = req[1];
                count++;
            }
        }
        System.out.println(count);

        // out.println("Hello World");
        out.flush();
    }

    void debug(Object... os) {
        System.err.println(Arrays.deepToString(os));
    }

    public static void main(String[] args) {
        new Main().run();
    }
}