Problem:
Score : 400 points

Problem Statement

There are N one-off jobs available. If you take the i-th job and complete it, you will earn the reward of B_i after A_i days from the day you do it.

You can take and complete at most one of these jobs in a day.

However, you cannot retake a job that you have already done.

Find the maximum total reward that you can earn no later than M days from today.

You can already start working today.

Constraints

All values in input are integers.

1 \leq N \leq 10^5

1 \leq M \leq 10^5

1 \leq A_i \leq 10^5

1 \leq B_i \leq 10^4

Input

Input is given from Standard Input in the following format:

N M
A_1 B_1
A_2 B_2
\vdots
A_N B_N

Output

Print the maximum total reward that you can earn no later than M days from today.

Sample Input 1

3 4
4 3
4 1
2 2

Sample Output 1

5

You can earn the total reward of 5 by taking the jobs as follows:

Take and complete the first job today. You will earn the reward of 3 after four days from today.

Take and complete the third job tomorrow. You will earn the reward of 2 after two days from tomorrow, that is, after three days from today.

Sample Input 2

5 3
1 2
1 3
1 4
2 1
2 3

Sample Output 2

10

Sample Input 3

1 1
2 1

Sample Output 3

0

Solution in Ruby (2.3.3) (judge verdict: Time Limit Exceeded)

N, M = gets.chomp.split(' ').map(&:to_i)

AB = []
N.times do |i|
  AB[i] = gets.chomp.split(' ').map(&:to_i)
end

group_by_day = AB.group_by{|v|v[0]}

day_reward = Array.new(M){[]}
group_by_day.each do |k, v|
  day_reward[k - 1] = v.map{|m| m[1]}.sort
end

#print "\nday_reward =>"
#print day_reward
#print "\n--------------------"

@output = 0
M.times do |i|
  arr = day_reward[i]
  # if arr.size == 0
  #   next
  # end
  candidate = []
  #print "\ncandidate=>"
  #print candidate
  (i+1).times do |t|
    candidate << day_reward[t].dup.pop if day_reward[t].size != 0
  end
  #print "\tcandidate=>"
  #print candidate
  if candidate.size != 0
    @output += candidate.max
    i.downto(0) do |t|
      if day_reward[t].dup.pop == candidate.max
        day_reward[t].pop 
        break
      end
    end
  
  end
  # if i < (M - 1)
  #   day_reward[i + 1] += arr
  # end
  #print "\t day_reward=>"
  #print day_reward
  #print "\t output=>"
  #print @output
end

#print "\n@output=>"
print @output