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.Text;
using System.Diagnostics;

namespace AtCoder
{
    class Program
    {
        private static string Read() { return Console.ReadLine(); }
        private static int ReadInt() { return int.Parse(Read()); }
        private static long ReadLong() { return long.Parse(Read()); }
        private static double ReadDouble() { return double.Parse(Read()); }
        private static int[] ReadInts() { return Array.ConvertAll(Read().Split(), int.Parse); }
        private static long[] ReadLongs() { return Array.ConvertAll(Read().Split(), long.Parse); }
        private static double[] ReadDoubles() { return Array.ConvertAll(Read().Split(), double.Parse); }

        static int Solve(int N, int M, Tuple<int, int>[] demands)
        {
            Array.Sort(demands);
            int cnt = 1;
            int min = demands[0].Item1;
            int max = demands[0].Item2;
            for (int i = 0; i < M; ++i)
            {
                min = Math.Max(demands[i].Item1, min);
                max = Math.Min(demands[i].Item2, max);
                if (min >= max)
                {
                    min = demands[i].Item1;
                    max = demands[i].Item2;
                    ++cnt;
                }
            }
            return cnt;
        }
        static void Main(string[] args)
        {
            int[] V1 = ReadInts();
            int N = V1[0];
            int M = V1[1];
            var demands = new Tuple<int, int>[M];
            for (int i = 0; i < M; ++i)
            {
                int[] V2 = ReadInts();
                demands[i] = new Tuple<int, int>(V2[0], V2[1]);
            }
            Console.WriteLine(Solve(N, M, demands));
        }
    }
}