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.Generic;
using System.Text;

namespace AtTest.ABC_103
{
    class D
    {
        static void Main(string[] args)
        {
            Method(args);
            Console.ReadLine();
        }

        static void Method(string[] args)
        {
            string[] input = Console.ReadLine().Split(' ');
            int n = int.Parse(input[0]);
            int m = int.Parse(input[1]);
            var request = new int[m][];
            for (int i = 0; i < m; i++)
            {
                string[] inText = Console.ReadLine().Split(' ');
                request[i] = new int[2];
                request[i][0] = int.Parse(inText[0]) - 1;
                request[i][1] = int.Parse(inText[1]) - 1;
            }

            List<int[]> check = new List<int[]>();
            Array.Sort(request, (a, b) => a[0] - b[0]);
            check.Add(request[0]);
            int nowIndex = 0;
            for (int i = 1; i < m; i++)
            {
                if (request[i][1] <= check[nowIndex][1])//内包
                {
                    check[nowIndex][0] = request[i][0];
                    check[nowIndex][1] = request[i][1];
                }
                else
                {
                    if (request[i][0] < check[nowIndex][1])//被り
                    {
                        check[nowIndex][0] = request[i][0];
                    }
                    else//独立
                    {
                        check.Add(request[i]);
                        nowIndex++;
                    }
                }
            }

            Console.WriteLine(check.Count);
        }
    }
}