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 ConsoleApp1
{
    class Program
    {
        static void Main(string[] args)
        {
            //input
            var nums = Console.ReadLine().Split(' ');
            int N = Convert.ToInt32(nums[0]);
            int M = Convert.ToInt32(nums[1]);
            List<Item> itemList = new List<Item>();
            for (int i = 0; i < N; i++)
            {
                var nums2 = Console.ReadLine().Split(' ');
                itemList.Add(new Item { Day = Convert.ToInt32(nums2[0]), Value = Convert.ToInt32(nums2[1]) });
            }
            itemList.Sort((x,y) => x.Day);
            int sum = 0;
            for(int i = 0; i < M; i++)
            {
                var a = itemList.Where(t => t.Day <= i + 1).OrderByDescending(t => t.Value).FirstOrDefault();
                if (a != null)
                {
                    sum += a.Value;
                    itemList.Remove(a);
                }
            }
            Console.WriteLine(sum);
        }
    }

    class Item
    {
        public int Day { get; set; }
        public int Value { get; set; }
    }
}