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: Runtime Error)

n, m = gets.split.map(&:to_i)
jobs = n.times.map{|i|
    gets.split.map(&:to_i)
}.sort_by{|it| it[1]}

income_of_day = [0] * m
free_day_count = income_of_day.size
while !jobs.empty? && free_day_count > 0
    j = jobs.pop
    index = m - j[0]
    while index >= 0 && income_of_day[index] != 0
        index -= 1
    end
    if index != -1
        free_day_count -= 1
        income_of_day[index] = j[1]
    end
end

puts income_of_day.inject(:+)