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: Time Limit Exceeded)

using System;

class Program
{
    static void Main(string[] args)
    {
        string[] array = Console.ReadLine().Split(' ');
        int N = int.Parse(array[0]);
        int M = int.Parse(array[1]);
        int[] a = new int[M];
        int[] b = new int[M];
        for (int i = 0; i < M; i++)
        {
            array = Console.ReadLine().Split(' ');
            a[i] = int.Parse(array[0]);
            b[i] = int.Parse(array[1]);
        }

        Insertion_sort(a, b);

        int brg = b[0];
        int bk = 1;

        for (int i = 0; i < M; i++)
        {
            if (a[i] >= brg)
            {
                brg = b[i];
                bk++;
            }
        }

        Console.WriteLine(bk);
    }

    static void Insertion_sort(int[] data2, int[] data)
    {
        for (int i = 1; i < data.Length; i++)
        {
            int tmp = data[i];
            int tmp2 = data2[i];
            if (data[i - 1] > tmp)
            {
                int j = i;
                do
                {
                    data[j] = data[j - 1];
                    data2[j] = data2[j - 1];
                    j--;
                } while (j > 0 && data[j - 1] > tmp);
                data[j] = tmp;
                data2[j] = tmp2;
            }
        }
    }
}