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;
using System.Text;

public class MainClass
{
	public const long Giri = 1000000007;
	public class Job
	{
		public int Day {get;set;}
		public int Reward {get;set;}
	}
	public static void Main(string[] args)
	{
		var p = Input().ToInt();
		var n = p[0];
		var m = p[1];
		
		var jobs = new Job[n];
		var dayHeap = new HeapQueue<Job>(n, (u,v)=>v.Day - u.Day);
		var rewHeap = new HeapQueue<Job>(n, (u,v)=>u.Reward - v.Reward);
		for(int i = 0; i < n; i++)
		{
			p = Input().ToInt();
			jobs[i] = new Job(){Day = p[0], Reward=p[1] };
			dayHeap.Push(jobs[i]);
		}
		long wa = 0;
		for(int i = 0; i <= m; i++)
		{
			for(;;)
			{
				if(dayHeap.Count == 0 || dayHeap.Peep().Day > i) break;
				var t = dayHeap.Pop();
				rewHeap.Push(t);
			}
			if(rewHeap.Count > 0)
			{
				var t = rewHeap.Pop();
				wa += t.Reward;
				
			}
		}
		Console.WriteLine(wa);
	}

	static long GCD(long a, long b)
	{
		var v = new[] { a, b };
		while (v[1] != 0) { v = new[] { v[1], v[0] % v[1] }; }
		return v[0];
	}
	public static string[] Input() { return Console.ReadLine().Split(' '); }
	public static int Clamp(int val, int max, int min = 0) { return Math.Min(max, Math.Max(min, val)); }
}

public static class Smarter
{
	public static int ToInt(this string str) { return int.Parse(str); }
	public static long ToLong(this string str) { return long.Parse(str); }
	public static int[] ToInt(this string[] str)
	{
		var r = new int[str.Length];
		for(int i = 0; i < str.Length; i++) r[i] = str[i].ToInt();
		return r;
	 }
}

class HeapQueue<T>
{
	private List<T> _buffer;
	private Comparison<T> _comparer;
	
	public HeapQueue()
	{
		_buffer = new List<T>();
	}
	
	public HeapQueue(int capacity, Comparison<T> comparer = null)
	{
		_buffer = new List<T>(capacity);

		_comparer = comparer;
	}
	
	public void Push(T el)
	{
		List<T> array = _buffer;
		int n = array.Count;
		array.Add(el);
		
		while (n != 0)
		{
			int i = (n - 1) / 2;
			if (Compare(array[n], array[i]) > 0)
			{
				T tmp = array[n]; array[n] = array[i]; array[i] = tmp;
			}
			n = i;
		}
	}
	
	public T Pop()
	{
		T el = _buffer[0];
		
		List<T> array = _buffer;
		
		int n = array.Count - 1;
		array[0] = array[n];
		array.RemoveAt(n);

		for (int i = 0, j; (j = 2 * i + 1) < n; )
		{
			if ((j != n - 1) && (Compare(array[j], array[j + 1]) < 0))
				j++;
			if (Compare(array[i], array[j]) < 0)
			{
				T tmp = array[j]; array[j] = array[i]; array[i] = tmp;
			}
			i = j;
		}
		return el;
	}
	
	public T Peep()
	{
		if(_buffer.Count <= 0)
			return default(T);

		return _buffer[0];
	}
	private int Compare(T left, T right)
	{
	/*
		if(_comparer == null)
			return left.CompareTo(right);
			*/
		return _comparer(left, right);
	}
	
	public int Count
	{
		get{ return _buffer.Count; }
	}
	
}