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: Wrong Answer)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApp1
{
    static class DSummerVacation
    {
        static void Main(string[] args)
        {
            var nm = Console.ReadLine().Split(' ').Select(x => int.Parse(x)).ToList();
            int n = nm[0];
            int m = nm[1];

            var abList = new List<Tuple<int, int>>();
            for (int i = 0; i < n; i++)
            {
                var ab = Console.ReadLine().Split(' ').Select(x => int.Parse(x)).ToList();
                int a = ab[0] - 1;
                int b = ab[1];
                abList.Add(Tuple.Create(b, a));
            }
            abList.Sort();
            abList.Reverse();

            long sum = 0;
            for (int i = m - 1; 0 <= i; i--)
            {
                foreach (var ab in abList)
                {
                    if (ab.Item2 <= i)
                    {
                        sum += ab.Item1;
                        abList.Remove(ab);
                        break;
                    }                    
                }
            }

            Console.WriteLine(sum);
            Console.ReadLine();
        }
    }
}