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.Comparator;
import java.util.Scanner;

public class Main {
    public static void main (String[] args) {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        int M = sc.nextInt();
        int[][] data = new int[M][2];
        for (int i = 0; i < M; i++) {
            data[i][0] = sc.nextInt();
            data[i][1] = sc.nextInt();
        }

        sortByColumn(data, 1);
        int result = 0;
        int nowclear = 0;

        int nowfrom = 0;
        int nowto = 0;
        for (int i = 0; i < M; i++) {
            nowfrom = data[i][0];
            nowto = data[i][1];

            if (nowclear < nowfrom) {
                nowclear = nowto - 1;
                result++;
            }
        }
        System.out.println(result);

    }

    //多维数组对某一列进行排序，参考https://www.cnblogs.com/whaozl/p/4061993.html
    private static void sortByColumn(int[][] ob, int column) {
        Arrays.sort(ob, new Comparator<int[]>() {
            public int compare(int[] one, int[] two) {
                return one[column] - two[column];
            }
        });
    }
}