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: Wrong Answer)

using System;
using System.Collections.Generic;
using System.Text;
using System.Numerics;
using System.IO;

namespace SolutionCS
{
    class Program
    {
        struct VAL
        {
            public int l;
            public int r;
        }


        static void Main(string[] args)
        {
            string[] s = Console.ReadLine().Split(' ');
            int N = int.Parse(s[0]);
            int M = int.Parse(s[1]);

            VAL[] v = new VAL[N];

            for (int i = 0; i < M; i++)
            {
                s = Console.ReadLine().Split(' ');
                int l = int.Parse(s[0]) - 1;
                int r = int.Parse(s[1]) - 1;

                v[l].r++;
                v[r].l++;
            }

            int ans = 0;
            int previsP = 0;
            for (int i = 0; i < N; i++)
            {
                if(previsP == 0)
                {
                    if(v[i].l > 0)
                    {
                        if(v[i].r > 0)
                        {
                            ans++;
                        }
                        else
                        {
                            previsP = 1;
                        }
                    }
                }
                else
                {
                    if(v[i].r > 0)
                    {
                        ans++;
                        previsP = 0;
                    }
                }
            }

            if(previsP != 0)
            {
                ans++;
            }

            Console.WriteLine(ans);
        }
    }
}