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
