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: Wrong Answer)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

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

        Youbou.OrderBy(w => -w.b);

        List<int> BreakBridge = new List<int>();

        for(int i = 0; i < Youbou.Length; i++)
        {
            bool IsBreake = false;
            for(int j = Youbou[i].b; Youbou[i].a > j; j--)
            {
                if(BreakBridge.Contains(j))
                {
                    IsBreake = true;
                    break;
                }
            }

            if(!IsBreake)
            {
                BreakBridge.Add(Youbou[i].b - 1);
            }
        }
        Console.WriteLine(BreakBridge.Count);
    }

    class War
    {
        public int a;
        public int b;

        
    }
}