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.Linq;//リストの使用
using System.Collections.Generic;
class Program
{
	static void Main()
	{
  int answer = 0;
  int lastPosition = -1;
	string[] input = Console.ReadLine().Split(' ');//Splitで区切り文字を指定して複数個受け取る。
	int n = int.Parse(input[0]);
	int m = int.Parse(input[1]);
  List<int[]> bridgeList = new List<int[]>();
    for(int i = 0; i < m; i++)
    {
    int[] nums = Array.ConvertAll(Console.ReadLine().Split(' '),int.Parse);//１行ごとに受け取る
    bridgeList.Add(new int[2]{nums[0]-1,nums[1]-1});
    }

    bridgeList.OrderBy(a => a[1]).ThenBy(a => a[0]);

    for(int i = 0; i < m; i++)
    {
      if(bridgeList[i][1] > lastPosition + 1 && bridgeList[i][0] > lastPosition)
      {
      answer++;
      lastPosition = bridgeList[i][1] - 1;
      }
    }
    
	Console.WriteLine(answer);//WriteLineをWriteとすると、改行なしで出力。
	}
}