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 Solution
{
    public struct Request
    {
        public int first;
        public int second;
    }

    public struct Problem
    {
        public int island;
        public int request;
        public List<Request> requests;
    }

    public class Parser
    {
        public static Problem Parse(List<string> lines)
        {
            var info = lines[0].Split(' ');
            var island = int.Parse(info[0]);
            var request = int.Parse(info[1]);

            var requests = new List<Request>();
            foreach (var line in lines.Skip(1))
            {
                var split = line.Split(' ');
                var req = new Request()
                {
                    first = int.Parse(split[0]),
                    second = int.Parse(split[1])
                };
                requests.Add(req);
            }

            return new Problem()
            {
                island = island,
                request = request,
                requests = requests
            };
        }
    }

    public class Solution
    {
        Problem problem;

        public Solution(Problem problem)
        {
            this.problem = problem;
        }

        public int Solve()
        {
            var bridge = 0;
            var requests = this.problem.requests.OrderBy(r => r.second).ToArray();
            while (requests.Length > 0)
            {
                var position = requests[0].second;
                bridge += 1;
                requests = requests.Where(r => r.first >= position).ToArray();
            }
            return bridge;
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            var lines = new List<string>();
            while (true)
            {
                var line = Console.ReadLine();
                if (line == null)
                {
                    break;
                }
                lines.Add(line);
            }
            var problem = Parser.Parse(lines);
            var solution = new Solution(problem);
            Console.WriteLine(solution.Solve());
        }
    }
}