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 Common Lisp (SBCL 1.1.14) (judge verdict: Runtime Error)

(let ((land-num (read))
      (request-num (read))
      (lands (make-array (1+ land-num))))
  (loop repeat request-num
        for land1 = (read)
        for land2 = (read)
        do (setf (aref lands land2) (max land1 (aref lands land2))))
  (loop with ans = 0
        for i from 1 upto land-num
        do (and (not (= 0 (aref lands i)))
                (= (aref lands (aref lands i)) ans)
                (incf ans))
           (setf (aref lands i) ans)
        finally (format t "~A~%" ans)))