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 Java8 (OpenJDK 1.8.0) (judge verdict: Accepted)

import java.util.Arrays;
import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        @SuppressWarnings("resource")
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int m = sc.nextInt();
        int[] a = new int[m];
        int[] b = new int[m];
        Pair2[] pairs = new Pair2[m];
        for (int i = 0; i < m; i++) {
            Pair2 p = new Pair2();
            p.from = sc.nextInt();
            p.end = sc.nextInt();
            pairs[i] = p;
        }
        Arrays.sort(pairs);

        int cnt = 0;
        int nowclear = 0;
        for (int i = 0 ; i < m ; i++) {
            int nowFrom = pairs[i].from;
            int nowEnd = pairs[i].end;
            if (nowclear < nowFrom) {
                cnt++;
                nowclear = nowEnd - 1;
            }
        }
        System.out.println(cnt);

    }



}

class Pair2 implements Comparable {
    int from;
    int end;

    @Override
    public int compareTo(Object other) {
        Pair2 otherpair = (Pair2) other;
        return end - otherpair.end;
    }
}