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.Text;
using System.Collections.Generic;
using System.Linq;

namespace D
{
    class Solver
    {
        public void Solve()
        {
            int[] nm = Console.ReadLine().Split(' ').Select(int.Parse).ToArray();
            int n = nm[0];
            int m = nm[1];
            var min = Enumerable.Repeat(15 << 26, n + 1).ToArray();
            for (int i = 0; i < m; i++)
            {
                int[] ab = Console.ReadLine().Split(' ').Select(int.Parse).ToArray();
                min[ab[0]] = Math.Min(min[ab[0]], ab[1]);
            }
            int p = 1, ans = 0;
            for (int i = n - 1; i >= 1; i--)
            {
                min[i] = Math.Min(min[i], min[i + 1]);
            }
            while (p < n && min[p] <= n)
            {
                p = min[p];
                ans++;
            }
            Console.WriteLine(ans);
        }

    }

    class M
    {
        public static void Main(string[] args)
        {
            new Solver().Solve();
        }
    }

};