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: Accepted)

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

class D
{
	static void Main()
	{
		var a = Console.ReadLine().Split().Select(int.Parse).ToArray();
		var js = Enumerable.Range(0, a[0]).Select(i => Console.ReadLine().Split().Select(int.Parse).ToArray()).Where(j => j[0] <= a[1]).GroupBy(j => j[0]).ToDictionary(g => g.Key, g => g.ToArray());

		var r = 0L;
		var l = new List<int>();
		for (var i = 1; i <= a[1]; i++)
		{
			if (js.ContainsKey(i))
				foreach (var j in js[i]) l.Insert(Search(l, j[1]), j[1]);

			if (!l.Any()) continue;
			r += l.Last();
			l.RemoveAt(l.Count - 1);
		}
		Console.WriteLine(r);
	}

	static int Search(IList<int> l, int v) => l.Any() ? Search(l, v, 0, l.Count) : 0;
	static int Search(IList<int> l, int v, int start, int count)
	{
		if (count == 1) return start + (v < l[start] ? 0 : 1);
		var c = count >> 1;
		var s = start + c;
		return v < l[s] ? Search(l, v, start, c) : Search(l, v, s, count - c);
	}
}