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: Wrong Answer)

import java.util.*;

public class Main {
	public static void main(String[] args) {
		// 自分の得意な言語で
		// Let's チャレンジ！！

		Scanner sc = new Scanner(System.in);
		String line = sc.nextLine();
		String s[] = line.split(" ", 0);
		int n = Integer.parseInt(s[0]);
		int m = Integer.parseInt(s[1]);
		ArrayList<Integer> c = new ArrayList<Integer>();
		ArrayList<Integer> b = new ArrayList<Integer>();
		for (int i = 0; i < m; i++) {
			line = sc.nextLine();
			String s2[] = line.split(" ", 0);
			c.add(Integer.parseInt(s2[0]));
			b.add(Integer.parseInt(s2[1]));
		}
		int a[];
		int ans = 0;

		while (c.size() != 0) {
			a = new int[n - 1];
			for (int i = 0; i < c.size(); i++) {
				for (int j = c.get(i) - 1; j < b.get(i) - 1; j++) {
					a[j]++;
				}
			}
			int max = 0;
			int pos = 0;
			for (int i = 0; i < n - 1; i++) {
				if (max < a[i]) {
					max = a[i];
					pos = i;
				}
			}
			for (int i = 0; i < c.size(); i++) {
				if (c.get(i) - 1 <= pos && b.get(i) - 1 > pos) {
					c.remove(i);

					b.remove(i);
					i--;
				}
			}
          ans++;
		}
      System.out.println(ans);

	}
}