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.Linq;
using System.Collections.Generic;

using E = System.Linq.Enumerable;

public class Program
{
    public static void Main()
    {
        var nm = Console.ReadLine().Split().Select(x => int.Parse(x)).ToArray();
        var n = nm[0];
        var m = nm[1];
        var lefts = new int[m];
        var rights = new int[m];
        for (int i = 0; i < m; i++)
        {
            var ab = Console.ReadLine().Split().Select(x => int.Parse(x)).ToArray();
            lefts[i] = ab[0];
            rights[i] = ab[1];
        }
        
        n += 1;
        var rside = new bool[n];
        var lside = new bool[n];
        for (int i = 0; i < m; i++)
        {
            rside[lefts[i]] = true;
            lside[rights[i]] = true;
        }
        var count = 0;
        int j = 1;
        while (j < n)
        {
            if (!rside[j])
            {
                j++;
                continue;
            }

            while (++j < n && !lside[j]);
            if (j < n && lside[j])
            {
                count++;
            }
        }
        Console.WriteLine(count);
    }
}