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 C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;


public class test
{	

    public static int ans = 0;
    public static int n;
    //public static int m;
    //public static int cnt;

    public static void Main()
	{
        int[] nums = Console.ReadLine().Split(' ').Select(s => int.Parse(s)).ToArray();
        int n = nums[0];
        int m = nums[1];

        int[] islands = new int[n];
        int[] A = new int[m];
        int[] B = new int[m];

        for(int i = 0 ; i < m ; i++)
        {
            nums = Console.ReadLine().Split(' ').Select(s => int.Parse(s)).ToArray();
            A[i] = nums[0];
            B[i] = nums[1];
        }
        Array.Sort(B,A);
        int c = 0;
        for(int i = 0 ; i < m ; i++)
        {
            if(A[i] < c){
            } else {
                islands[B[i]-1] = 1;
                c = B[i];
            }
        }
        ans = islands.Sum();
        Console.WriteLine(ans);
 
        
    }
}