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;
using System.Linq.Expressions;
using System.IO;
using System.Text;
using System.Diagnostics;

class Program
{
    static void Main()
    {
        int[] nm = Console.ReadLine().Split(' ').Select(int.Parse).ToArray();
        int n = nm[0];
        int m = nm[1];
        int[][] ab = Enumerable.Repeat(0, m).Select(_ => Console.ReadLine().Split(' ').Select(int.Parse).ToArray()).ToArray();
        List<Tuple<int, int>> d = new List<Tuple<int, int>>();

        for (int i = 0; i < m; i++)
        {
            d.Add(new Tuple<int, int>(ab[i][0], ab[i][1]));
        }

        d.Sort((a, b) => (a.Item2 != b.Item2) ? (a.Item2 - b.Item2) : (a.Item1 - b.Item1));

        int ans = 0;
        int destroy = 0;

        for(int i = 0; i < m; i++)
        {
            if(destroy <= d[i].Item1)
            {
                destroy = d[i].Item2;
                ans++;
            }
        }

        Console.WriteLine(ans);
    }
}