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 C# (Mono 4.6.2.0) (judge verdict: Time Limit Exceeded)

using System;
using System.Collections.Generic;
using System.Linq;

namespace AtCoder
{
    class Program
    {
        static void Main(string[] args)
        {
            //[summary]D - Summer Vacation
            var line = ReadLine();
            int n = line[0];
            int m = line[1];
            var jobs = new List<Job>();

            for(int i = 0; i < n; i++)
            {
                line = ReadLine();

                if (line[0] <= m)
                {
                    var job = new Job(line[0], line[1]);
                    jobs.Add(job);
                }
            }

            if (jobs.Count == 0)
            {
                Console.WriteLine(0);
            }
            else
            {
                jobs = jobs.OrderByDescending(x => x.Money).ToList();
                long reward = 0;

                for (int i = 1; i <= m; i++)
                {
                    int index = GetIndex(jobs, i);

                    if (index >= 0)
                    {
                        reward += jobs[index].Money;
                        jobs.RemoveAt(index);
                    }
                }

                Console.WriteLine(reward);
            }            
        }

        static int GetIndex(List<Job> jobs,int days)
        {
            for(int i = 0; i < jobs.Count; i++)
            {
                if (jobs[i].Day <= days)
                {
                    return i;
                }
            }

            return -1;
        }

        static List<int> ReadLine()
        {
            var line = Console.ReadLine();
            var array = line.Split(' ');

            return array.Select(x => int.Parse(x)).ToList();
        }
    }

    struct Job
    {
        public int Day { set; get; }
        public int Money { set; get; }

        public Job(int day, int money)
        {
            Day = day;
            Money = money;
        }
    }
}