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: Accepted)

n,m=gets.chomp.split.map { |e| e.to_i  }

a=Array.new(m,0)
b=Array.new(m,0)


m.times do |i|

  a[i],b[i]=gets.chomp.split.map { |e| e.to_i  }

end

c=Array.new(m)


for i in 0..m-1
  c[i] = {'a' => a[i],'b' => b[i]}
end

def quick_sort_hash(c)
  if c==[]
    return []
  end

  smaller = c.select{ |h| h["b"] < c[0]["b"] }
  same = c.select{ |h| h["b"] == c[0]["b"] }
  larger = c.select { |h| h["b"] > c[0]["b"]  }

  return quick_sort_hash(smaller) + same + quick_sort_hash(larger)

end

e=quick_sort_hash(c)

x=-1
dp = 0

m.times do |i|
  if e[i]["a"] >= x
    dp +=1
    x=e[i]["b"]
  end
end

puts dp