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.Linq;
using System.Collections.Generic;
using System.Numerics;
using static System.Console;

class Program {
    internal static void Main(string[] args) {
        var inp = ReadLine().Split(' ').Select(s => int.Parse(s)).ToArray();
        var N = inp[0];
        var M = inp[1];
        var ab = ReadLine().Split(' ').Select(s => int.Parse(s)).ToArray();
        var min = ab[0];
        var max = ab[1];

        var bri = new List<int[]>();
        bri.Add(new int[]{min, max});

        var ans = 1;
        for (int i = 0; i < M - 1; i++) {
            ab = ReadLine().Split(' ').Select(s => int.Parse(s)).ToArray();
            var add = true;

            foreach (int[] hoge in bri) {
                if ((hoge[0] <= ab[0] && ab[0] < hoge[1]) ||
                    (hoge[0] <= ab[1] && ab[1] <= hoge[1])) {
                    if (hoge[0] < ab[0]) { hoge[0] = ab[0]; }
                    if (ab[1] < hoge[1]) { hoge[1] = ab[1]; }
                    add = false;
                    break;
                }
            }

            if (add) {
                ans++;
                bri.Add(new int[] { ab[0], ab[1] });
            }
        }

        WriteLine(ans);
    }

}