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: Time Limit Exceeded)

"
ABC 103 D - Islands War
https://atcoder.jp/contests/abc103/tasks/abc103_d
"

# 要求の構造体
Req = Struct.new(:s, :t)

# 入力
N, M = gets.split.map(&:to_i)
a = []
b = []
r = [] #構造体の配列
M.times do |i|
  a[i], b[i] = gets.split.map(&:to_i)
  r << Req.new(a[i], b[i])
end

# 配列rをtの値で昇順ソート
r = r.sort {
  |r1, r2| r1.t <=> r2.t
}

# greedy
n = r.shift.t # tのうち最も小さいもの
ans = 1
while not r.empty?
  # s が t より大きいなら，そのsに関してはnの前の橋を切っても分離されない
  if r.first.s < n
    r.delete(r.first)
  else
    n = r.shift.t
    ans += 1
  end
end
puts ans