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;

class Program
{
    #region library
    static int n = 0;
    static int r = 0;
    static int c = 0;
    static int x = 0;
    static int y = 0;
    static string s = "";
    static string[] ss;
    static long sum = 0;
    static long cnt = 0;
    static long max = -1;
    static long min = 2147483647;
    static bool flg = false;
    static bool flg2 = false;
    static string alpha = "abcdefghijklmnopqrstuvwxyz";
    static int[] arr;
    static char[,] map;
    static int[,] imap;
    static string rl()
    {
        return Console.ReadLine();
    }
    static string[] rls()
    {
        return Console.ReadLine().Split();
    }
    static void wl(object obj = null)
    {
        Console.WriteLine(obj);
    }
    static void w(object obj)
    {
        Console.Write(obj);
    }
    static int Int(string s)
    {
        return int.Parse(s);
    }
    static void yes()
    {
        wl("YES");
    }
    static void no()
    {
        wl("NO");
    }
    static void Sum()
    {
        wl(sum);
    }
    static void Cnt()
    {
        wl(cnt);
    }
    static void setarray(int num = -1)
    {
        if (n == 0)
        {
            n = Int(rl());
        }
        arr = new int[n];
        if (num == -1) ss = rls();
        for (int i = 0; i < n; i++)
        {
            if (num == -1)
                arr[i] = Int(ss[i]);
            else
                arr[i] = num;
        }
    }
    static void setmap(char def = '@')
    {
        if (r == 0 && c == 0)
        {
            ss = rls();
            r = Int(ss[0]);
            c = Int(ss[1]);
        }
        map = new char[r, c];
        for (int i = 0; i < r; i++)
        {
            if (def == '@') s = rl();
            for (int j = 0; j < c; j++)
            {
                map[i, j] = (def == '@') ? s[j] : def;
            }
        }
    }
    static void setimap(int def = -1)
    {
        if (r == 0 && c == 0)
        {
            ss = rls();
            r = Int(ss[0]);
            c = Int(ss[1]);
        }
        imap = new int[r, c];
        for (int i = 0; i < r; i++)
        {
            if (def == -1) s = rl();
            for (int j = 0; j < c; j++)
            {
                imap[i, j] = (def == -1) ? Int(s[j].ToString()) : def;
            }
        }
    }
    #endregion

    static void Main(string[] args)
    {
        ss = rls();
        n = Int(ss[0]);
        int m = Int(ss[1]);
        int[] bridges = new int[n];
        string[] desires = new string[m];
        for (int i = 0; i < m; i++)
        {
            ss = rls();
            desires[i] = ss[0] + " " + ss[1];
        }
        Array.Sort(desires);

        for (int i = 0; i < m; i++)
        {
            ss = desires[i].Split();
            int start = Int(ss[0]);
            int end = Int(ss[1]);

            flg = false;
            for (int j = start - 1; j < end; j++)
            {
                if (bridges[j] == -1)
                {
                    flg = true;
                    break;
                }
            }
            if (!flg)
            {
                cnt++;
                bridges[end - 2] = -1;
            }
        }

        Cnt();
    }
}