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.Collections;
using System.Linq; 
using E = System.Linq.Enumerable; 


class Program {
    static void Main(string[] args) {
        
        var NM = Console.ReadLine().Split(' ').Select(x => int.Parse(x)).ToArray();
        var N = NM[0];
        var M = NM[1];
        
        var isl = new int[N+1];
        
        for(int i = 0;i < N+1 ;i++){
            isl[i] = N+1;
        }
       // 
        for(int i = 0;i < M ;i++){
            var AB = Console.ReadLine().Split(' ').Select(x => int.Parse(x)).ToArray();
            var A = AB[0];
            var B = AB[1];
            if(isl[A] > B){
                isl[A] = B;
            }
        }
        //Console.WriteLine(22);
        var tmp = N+1;
        var ans = 0;
        for(int i = N;i > 0 ;i--){
            if(isl[i] < tmp){
                tmp = i+1;
                ans++;
            }
        }
        
        Console.WriteLine(ans);
     }
     
}
     
     
                   
public class PriorityQueue<T> : IEnumerable<T> { private readonly List<T> _data = new List<T>(); private readonly IComparer<T> _comparer; private readonly bool _isDescending; public PriorityQueue(IComparer<T> comparer, bool isDescending = true) { _comparer = comparer; _isDescending = isDescending; } public PriorityQueue(Comparison<T> comparison, bool isDescending = true) : this(Comparer<T>.Create(comparison), isDescending) { } public PriorityQueue(bool isDescending = true) : this(Comparer<T>.Default, isDescending) { } public void Enqueue(T item) { _data.Add(item); var childIndex = _data.Count - 1; while (childIndex > 0) { var parentIndex = (childIndex - 1) / 2; if (Compare(_data[childIndex], _data[parentIndex]) >= 0) break; Swap(childIndex, parentIndex); childIndex = parentIndex; } } public T Dequeue() { var lastIndex = _data.Count - 1; var firstItem = _data[0]; _data[0] = _data[lastIndex]; _data.RemoveAt(lastIndex--); var parentIndex = 0; while (true) { var childIndex = parentIndex * 2 + 1; if (childIndex > lastIndex) break; var rightChild = childIndex + 1; if (rightChild <= lastIndex && Compare(_data[rightChild], _data[childIndex]) < 0) childIndex = rightChild; if (Compare(_data[parentIndex], _data[childIndex]) <= 0) break; Swap(parentIndex, childIndex); parentIndex = childIndex; } return firstItem; } public T Peek() { return _data[0]; } private void Swap(int a, int b) { var tmp = _data[a]; _data[a] = _data[b]; _data[b] = tmp; } private int Compare(T a, T b) { return _isDescending ? _comparer.Compare(b, a) : _comparer.Compare(a, b); } public int Count => _data.Count; public IEnumerator<T> GetEnumerator() { return _data.GetEnumerator(); } IEnumerator IEnumerable.GetEnumerator() => GetEnumerator(); }