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);
        final int N = sc.nextInt();
        int M = sc.nextInt();
        int[] l1 = new int[N+1];
        int ans = 0;
        for(int i = 1;i < N+1;i++)  l1[i] = -1;
        for(int i = 0;i < M;i++){
            int ai = sc.nextInt();
            int bi = sc.nextInt();
            l1[bi] = Math.max(ai, l1[bi]);
        }
        int right = N;
        int left = 0;
        while(true){
            if(right == 1)  break;
            if(l1[right] > 0){
                left = l1[right];
            }else{
                right--;
                continue;
            }
            while(true){
                if(left == right){
                    ans++;
                    break;
                }
                left = Math.max(left, l1[right]);
                right--;
            }
        }
        System.out.println(ans);
    }
}