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;

namespace Test01
{
    struct AB
    {
        public int a;
        public int b;
        public AB(int a, int b)
        {
            this.a = a;
            this.b = b;
        }
    }
    class Program
    {
        static void Main(string[] args)
        {
            //入力1(N, M)
            int[] input = Console.ReadLine().Split(' ').Select(x => int.Parse(x)).ToArray();
            int N = input[0];
            int M = input[1];
            //入力2(ab)
            AB[] ab = new AB[M];
            for(int i = 0; i < M; i++)
            {
                input = Console.ReadLine().Split(' ').Select(x => int.Parse(x)).ToArray();              
                ab[i] = new AB(input[0], input[1]);
            }

            int cnt = 0;  //切った数をカウント
            int cut_e = -1;  //直前に切った橋の番号
            foreach(var s in ab.OrderBy(x => x.b))
            {
                if (s.a > cut_e) { cut_e = s.b - 1; cnt++; }
            }

            Console.WriteLine(cnt);
        }
    }
}