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 Ruby (2.3.3) (judge verdict: Wrong Answer)

n,m = gets.split.map(&:to_i)

arr1 = []

m.times{
    
    arr1<<gets.split.map(&:to_i)}
    


arr1 = arr1.sort

i=1
(m-1).times{
    
    
    if arr1[i][0] == arr1[i-1][0]
        
        arr1.delete_at(i)
        
    elsif   arr1[i][1] <= arr1[i-1][1]
        arr1.delete_at(i-1)
        
    else
        i+=1
    end
    
    
    
}





now = 0
ll = arr1.length
j = 0
ans = 0

loop{
    

    
     ans+=1
    
    
    now=arr1[j][1]
    
    
    
    loop{
        
        if arr1[j+1] && arr1[j+1][0]<now
            j+=1
    
        
       else
           
           break
       end
        
        
        
    }
    
    
    
     j+=1
    
    if j>=ll
      break
     end
    
    
   
    
    
    
 }


puts ans