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();
        List<Pair> rl = new ArrayList<Pair>();
        for (int i = 0; i < m; i++) {
            int a = sc.nextInt();
            int b = sc.nextInt();
            rl.add(new Pair(b, a));
        }
        Collections.sort(rl);
        int ans = 0;
        int start = 0;
        for (Pair p : rl) {
            if (p.second >= start) {
                ans++;
                start = p.first;
            }
        }
        System.out.println(ans);
    }
}
class Pair implements Comparable<Pair> {
    int first;
    int second;
    public Pair(int i, int j) {
        this.first = i;
        this.second = j;
    }
    public int compareTo(Pair p) {
        if (this.first < p.first) return -1;
        else if (this.first > p.first) return 1;
        else {
            if (this.second < p.second) return -1;
            else if (this.second > p.second) return 1;
            else return 0;
        }
    }
}