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.*;

class Pair implements Comparable {
    int left;
    int right;

    Pair(int left, int right) {
        this.left = left;
        this.right = right;
    }
    public int compareTo(Object o) {
        Pair other = (Pair) o;
        return this.right - other.right;
    }
}

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        int M = sc.nextInt();

        Pair[] pairs = new Pair[M];
        for (int i = 0; i < M; i++) {
            int left = sc.nextInt() - 1;
            int right = sc.nextInt() - 1;
            pairs[i] = new Pair(left, right);
        }

        Arrays.sort(pairs);

        int lastBride = -1;
        int ret = 0;
        for (int i = 0; i < M; i++) {
            Pair pair = pairs[i];
            if (pair.left > lastBride) {
                ret++;
                lastBride = pair.right - 1;
            }
        }

        System.out.println(ret);
    }
}