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)

=begin 

a = gets.split.map(&:to_i)
a.sort!
sum = 0

for i in  0..a.length-1 do
  if i == (a.length-1)
    puts sum
    break
  end
  sum += a[i+1] - a[i]
end
s = gets.chomp
t = gets.chomp
ans = false

for i in 0..s.length-1 do
  new_st = s[i+1..s.length-1] + s[0..i]
  if new_st == t
    ans = true
    break
  end 
end

puts (ans ? 'Yes' : 'No')
=end

n, m = gets.split.map(&:to_i)
pairs = []
m.times do |i|
  pairs << gets.split.map(&:to_i)
end

hash = {}

ite = 0
pairs.each do |pair|
  a = pair[0]
  b = pair[1]
  if hash[a].nil?
    hash[a] = []
    hash[a] << ite
  else
    hash[a] << ite
  end
  if hash[b].nil?
    hash[b] = []
    hash[b] << ite
  else
    hash[b] << ite
  end
  ite += 1
end

temp_arr = []
count = 0
n.times do |i|
  if hash[i+1].nil?
    next
  end
  if i == n-1
    next
  end
  hash[i+1].each do |j|
    if temp_arr.include?(j)
      count += 1
      temp_arr = []
      next
    else
      temp_arr << j
    end
  end
end

puts count