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.*;
public class Main {
		
	public static void main(String[] args) {		
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		int M = sc.nextInt();
		int war[][] = new int[M][2];
		for(int i=0;i<M;i++) {
			int a = sc.nextInt() - 1;
			int b = sc.nextInt() - 1;
			if(a > b) {
				int tmp = a;
				a = b;
				b = tmp;
			}
			war[i][0]=a;
			war[i][1]=b;
		}
		sc.close();
		
		//bで昇順にソート
		Arrays.sort(war, (w1,w2) -> w1[1] - w2[1] );
		int head = 0; 
		int cnt = 0;
		for(int i=0;i<M;i++) {
			int a = war[i][0];
			int b = war[i][1];
			if(a >= head) {
				cnt ++;
				head = b;
			}
		}
		System.out.println(cnt);
	}
}