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)

ret = Array.new(m, 0) # [最終日, .... , 最初の日]

inputs = []
(0..(n-1)).each do |i|
    a,b=gets.chomp.split(" ").map(&:to_i)
    inputs[i] = [a, b]
end

inputs = inputs.sort {|a, b| b[1] <=> a[1] }

inputs.each do |input|
    a = input[0]
    b = input[1]
    if m >= a
        ((a-1)..m-1).each do |i| 
            if ret[i] == 0
                ret[i] = b
                break
            end
        end
    end
end

puts ret.reduce(&:+)