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.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.Collections;

public class Main {

	public static void main(String[] args) throws NumberFormatException, IOException {

		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

		String[] nm = br.readLine().split(" ");
		int n = Integer.parseInt(nm[0]);
		int m = Integer.parseInt(nm[1]);

		ArrayList<String> demands = new ArrayList<String>(m);

		for (int i = 0; i < m; i++) {
			String[] miline = br.readLine().split(" ");
			demands.add(String.format("%05d", Integer.parseInt(miline[0])) + "-"
					+ String.format("%05d", Integer.parseInt(miline[1])));
		}

		Collections.sort(demands);

		int ans = 0;
		int min = -1;
		int max = 999999999;

		for (int i = 0; i < m; i++) {
			String[] ab = demands.get(i).split("-");
			int a = Integer.parseInt(ab[0]);
			int b = Integer.parseInt(ab[1]);
			if (a < max) {
				min = Math.max(a, min);
				max = Math.min(b, max);
			} else {
				ans++;
				min = a;
				max = b;
			}
		}

		ans++;

		System.out.println(ans);

	}

}