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)

def main
  islands_num, req_num = ARGF.gets.split.map(&:to_i)

  # ソートしない解法 ※islands_num分のArrayを最初から用意しておくだけ
  requests = Array.new(islands_num)
  req_num.times do
    req = ARGF.gets.split.map(&:to_i)
    requests[req[1]] = req
  end

  # 切断した場所 例）1の場合、1-2 の間を切断したという意味
  last = 0
  cnt = requests.inject(0) do |cnt, req|
    if !req.nil? && last < req[0]
      last = req[1] - 1
      cnt += 1
    end
    cnt
  end

  puts cnt
end

if __FILE__ == $0
  main
end