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: Runtime Error)

import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;


public class Main {

    public static void main(String[] args) {

        Scanner scanNumber = new Scanner(System.in);

        int howIland = scanNumber.nextInt();

        int howWar = scanNumber.nextInt();

        int[] ilandEnemyA;
        ilandEnemyA = new int[howWar];

        int[] ilandEnemyB;
        ilandEnemyB = new int[howWar];

        for(int i=0; i <= howWar-1 ; i++){

            ilandEnemyA[i] = scanNumber.nextInt();
            ilandEnemyB[i] = scanNumber.nextInt();

        }

        scanNumber.close();

        int howNeedBreage = 0;

        List<Integer> ilands = new ArrayList<Integer>();

        for(int i=0; i <= howIland ; i++){
            boolean canBreak = false;

            for(int j=0; j <= howWar-1 || canBreak ; j++){
                if(ilands.contains(ilandEnemyA[j]) && ilands.contains(ilandEnemyB[j])){
                    howNeedBreage++;
                    ilands.clear();
                    ilands.add(i);
                    canBreak = true;
                }
            }

            ilands.add(i+1);
        }
        System.out.print(howNeedBreage);

    }
}