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.*;

import static java.lang.System.*;

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

        PriorityQueue<Entry> pq = new PriorityQueue<Entry>(N, new EntryComparator());
        for (int i = 0; i < M; i++) {
            Entry entry = new Entry(sc.nextInt(), sc.nextInt());
            pq.add(entry);
        }

        Entry entry = pq.poll();
        int maxB = entry.b;

        int ans = 1;
        while(!pq.isEmpty()){
            entry = pq.poll();
            if(entry.a >= maxB){
                ans++;
                maxB = entry.b;
            } else {
                maxB = Math.min(entry.b, maxB);
            }
        }

        out.println(ans);
    }

    // 昇順
    static class EntryComparator implements Comparator<Entry> {
        @Override
        public int compare(Entry t1, Entry t2) {
            if (t1.a >= t2.a){
                return 1;
            }
            return -1;
        }
    }

    static class Entry {
        public int a;
        public int b;

        public Entry(int a, int b){
            this.a = a;
            this.b = b;
        }
    }
}