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.Globalization;
using System.Linq;
using System.Text;
using System.IO;

namespace Contest
{
    class Scanner
    {
        private string[] line = new string[0];
        private int index = 0;

        public string Next()
        {
            if (line.Length <= index)
            {
                line = Console.ReadLine().Split(' ');
                index = 0;
            }

            var res = line[index];
            index++;
            return res;
        }

        public int NextInt()
        {
            return int.Parse(Next());
        }

        public long NextLong()
        {
            return long.Parse(Next());
        }

        public string[] Array()
        {
            line = Console.ReadLine().Split(' ');
            index = line.Length;
            return line;
        }

        public int[] IntArray(int n)
        {
            var array = Array();
            var result = new int[n];
            for (int i = 0; i < n; i++)
            {
                result[i] = int.Parse(array[i]);
            }

            return result;
        }

        public int[] IntArray()
        {
            var array = Array();
            var result = new int[array.Length];
            for (int i = 0; i < array.Length; i++)
            {
                result[i] = int.Parse(array[i]);
            }

            return result;
        }


        public long[] LongArray()
        {
            var array = Array();
            var result = new long[array.Length];
            for (int i = 0; i < array.Length; i++)
            {
                result[i] = long.Parse(array[i]);
            }

            return result;
        }

        public uint NextUint()
        {
            return uint.Parse(Next());
        }
    }

    struct S
    {
        public int A, B;

        public S(int a, int b)
        {
            A = a;
            B = b;
        }
    }

    class Program
    {
        private int N, M;
        private S[] S;
        private void Scan()
        {
            var sc = new Scanner();
            N = sc.NextInt();
            M = sc.NextInt();
            S = new S[M];
            for (int i = 0; i < M; i++)
            {
                S[i] = new S(sc.NextInt() - 1, sc.NextInt() - 1);
            }
        }
        public void Solve()
        {
            Scan();
            Array.Sort(S, (a, b) =>
            {
                if (a.A == b.A)
                {
                    return -a.B.CompareTo(b.B);
                }
                else
                {
                    return a.A.CompareTo(b.A);
                }
            });
            int left = -1;
            int right = N;
            int cnt = 0;
            bool flag = true;
            //Console.WriteLine();
            for (int i = 0; i < M; i++)
            {
                if (left < S[i].A)
                {
                    left = S[i].A;
                }
                right = Math.Min(right, S[i].B);
                if (right <= left)
                {
                    cnt++;
                    left = S[i - 1].A + 1;
                    right = N;
                    flag = false;
                    continue;
                }

                flag = true;
            }

            if (right > left && flag) cnt++;
            Console.WriteLine(cnt);
        }

        static void Main() => new Program().Solve();
    }
}