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;
class Program
{
    static void Main(string[] args)
    {
        string[] NM = Console.ReadLine().Split(' ');
        int N = int.Parse(NM[0]);
        int M = int.Parse(NM[1]);
        List<Tuple<int, int>> n = new List<Tuple<int, int>>();
        for (var i = 0; i < N; i++)
        {
            string[] n2 = Console.ReadLine().Split(' ');
            n.Add(new Tuple<int, int>(int.Parse(n2[1]), int.Parse(n2[0])));
        }
        n.Sort();
        n.Reverse();
        long ans = 0;
        List<int> days = new List<int>();
        for (var j = M-1; j >= 0; j--)
        {
          days.Add(j);
        }        
        foreach (var i in n)
        {
            int hit = -1;
            foreach (var j in days)
            {
              if (M-j < i.Item2) continue;
              hit = j;
              break;
            }
            if (hit >= 0)
            {
              ans += i.Item1;
              days.Remove(hit);
              if (days.Count() == 0) break;
            }
        }
        Console.WriteLine(ans);
    }
}