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) {
		Scanner scanner = new Scanner(System.in);
		int n = scanner.nextInt();
		int m = scanner.nextInt();
		int arr[][] = new int[m][2];
		for (int i=0; i<m; i++){
			arr[i][0] = scanner.nextInt();
			arr[i][1] = scanner.nextInt();
		}

		arr = sort(arr);

		int ans = 0;
		int begin = 0;
		int end = n;
		for(int i=0; i<m; i++){
			begin = Math.max(begin, arr[i][0]);
			end = Math.min(end, arr[i][1]);
			if (begin>=end) {
				ans++;
				begin = arr[i][0];
				end = arr[i][1];
			}
		}
		ans++;
		System.out.println(ans);

	}

	static int[][] sort(int[][] arr){
		Arrays.sort(arr, (o1, o2) -> {
			if (o1[0] == o2[0]) {
				return o1[1] - o2[1];
			} else {
				return o1[0] - o2[0];
			}
		});
		return arr;
	}

}