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: Runtime Error)

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

namespace AtCoderD
{
    class Program
    {
        static void Main(string[] args)
        {
            List<Int64[]> list = new List<Int64[]>();

            for (; ; )
            {
                string line = Console.ReadLine();
                if (line == "")
                {
                    break;
                }
                string[] pair = line.Split(' ');
                Int64 a = Int64.Parse(pair[0]);
                Int64 b = Int64.Parse(pair[1]);
                if (a<b)
                {
                    list.Add(new Int64[] { a, b });
                }
                else
                {
                    list.Add(new Int64[] { b, a });
                }
                System.Console.WriteLine("a=" + list.Last()[0] + " b=" + list.Last()[1]);
            }
            System.Console.WriteLine("list.Count()=" + list.Count());

            int limit = 100;
            bool change = true;
            while (limit > 0 && change)
            {
                change = false;
                for (int i= list.Count(); 0<=i; i--)
                {
                    Int64[] a = list[i - 1];
                    Int64[] b = list[i];

                    if (a[0]<=b[0] && b[1]<=a[1])
                    {
                        a[0] = b[0];
                        a[1] = b[1];
                    }

                    if (list[i-1][0]==list[i][0])
                    {
                        list.RemoveAt(i);
                    }
                    change = true;
                }

                limit--;
            }

            //System.Console.WriteLine("list.Count()=" + list.Count());
            System.Console.WriteLine(list.Count());
            //Console.ReadKey();
        }
    }
}