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;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApp15
{
    class Class217
    {
        static void Main()
        {
            var NM = Console.ReadLine().Split().Select(int.Parse).ToArray();
            int N = NM[0], M = NM[1];
            var MIN = new int[M];
            var MAX = new int[M];
            for(int  i = 0; i < M; i++)
            {
                var ab = Console.ReadLine().Split().Select(int.Parse).ToArray();
                int a = ab[0], b = ab[1];
                MIN[i] = a;
                MAX[i] = b;
            }
            Array.Sort(MAX,MIN);
            int edge = int.MinValue;
            int count = 0;
            for(int i = 0; i < M; i++)
            {
                if (edge <= MIN[i])
                {
                    count++;
                    edge = MAX[i];
                }
            }
            Console.WriteLine(count);
        }
    }
}