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# (.NET Core 3.1.201) (judge verdict: Accepted)

using System;
using System.Linq;
using System.Collections.Generic;

namespace ABC
{
    class ProgramD
    {
        struct Bridge
        {
            public int a;
            public int b;
        }
        static void Main(string[] args)
        {
            var inputs = Console.ReadLine().Split(" ");
            var n = int.Parse(inputs[0]);
            var m = int.Parse(inputs[1]);

            var list = new Bridge[m];
            for (var i = 0; i < m; i++)
            {
                var inputs1 = Console.ReadLine().Split(" ");
                var a = int.Parse(inputs1[0]) - 1;
                var b = int.Parse(inputs1[1]) - 1;
                list[i] = new Bridge() { a = a, b = b };
            }

            var rList = Enumerable.Range(0, n - 1).Select(x => true).ToArray();
            list = list.OrderBy(x => x.b).ToArray();

            var result = 0;
            foreach (var b in list)
            {
                var isPath = true;
                for (var i = b.a; i < b.b; i++)
                {
                    if (!rList[i])
                    {
                        isPath = false;
                        break;
                    }
                }

                if (isPath)
                {
                    rList[b.b - 1] = false;
                    result++;
                }
            }

            Console.WriteLine(result);
        }
    }
}