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.Linq;

namespace D
{
    class Program
    {
        static void Main(string[] args)
        {
            int[] NM = Console.ReadLine().Split().Select(i => Convert.ToInt32(i)).ToArray();
            List<Tuple<int, int>> req = Enumerable.Range(0, NM[1]).Select(i =>
            {
                int[] ab = Console.ReadLine().Split().Select(j => Convert.ToInt32(j)).ToArray();
                return new Tuple<int, int>(ab[0], ab[1]);
            }).OrderByDescending(i => i).ToList();
            int ans = 0;
            int edge = NM[0] + 1;
            req.ForEach(i => {
                if (i.Item1 < edge && i.Item2 < edge)
                {
                    ans++;
                    edge = i.Item1;
                }
            });
            Console.WriteLine(ans);
        }
    }

}