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.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ProblemR
{
    class Program
    {
        static void Main(string[] args)
        {
            var read = Console.ReadLine().Split(' ').Select(x => int.Parse(x)).ToArray();
            var N = read[0];
            var M = read[1];

            var xs = new int[M];
            var ys = new int[M];

            for (int i = 0; i < M; i++)
            {
                read = Console.ReadLine().Split(' ').Select(x => int.Parse(x)).ToArray();
                xs[i] = read[0];
                ys[i] = read[1];
            }
            var sortedInd = ys.Select((y, i) => new { ind = i, point = y }).OrderBy(x => x.point).Select(x => x.ind);
            var sortedxs = sortedInd.Select(i => xs[i]).ToArray();
            var sortedys = sortedInd.Select(i => ys[i]).ToArray();

            var current = 1;
            var res = 0;
            for (int i = 0; i < sortedxs.Length; i++)
            {
                if(sortedxs[i] >= current)
                {
                    current = sortedys[i];
                    res++;
                }
            }
            Console.WriteLine(res);
        }
    }
}