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.Linq;
 
static class MainClass {
  //static Action yes = Yes;
  //static Action no = No;
  public struct arch{
    public long a;
    public long b;
    public arch(long _a, long _b){
      a = _a;
      b = _b;
    }
  }

  public static void Main (string[] args) {
    var temp = Console.ReadLine().Split(' ').Select(long.Parse);
    var N = temp.ElementAt(0);
    var M = temp.ElementAt(1);
    var archList = new List<arch>();
    var ans = (long)0;
    var breakArch = (long)0;
    for(var i = 0; i < M; i++){
      var temp2 = Console.ReadLine().Split(' ').Select(long.Parse);
      archList.Add(new arch(temp2.ElementAt(0), temp2.ElementAt(1)));
    }
    archList = archList.OrderBy(x => x.b).ToList();

    archList.ForEach(x => {
      if(ans == 0 || breakArch < x.a){
        breakArch = x.b - 1;
        ans++;
      }

    });
    
    Console.WriteLine(ans);

  }
  
}