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 CSharpSample01
{
    
    class PriorityQueue
    {
        int H;
        int[] heap;
        const int MAX = 2000000;

        public PriorityQueue()
        {
            H = 0;
            heap = new int[MAX];
        }
        
        public PriorityQueue(int size)
        {
            H = size;
            heap = new int[H + 1];
        }

        public void MaxHeapfy(int p)
        {
            while(2 * p <= H)
            {
                int son_n = 2 * p;
                int son = heap[2 * p];
                if(2 * p + 1 <= H && son < heap[2 * p + 1])
                {
                    son = heap[2 * p + 1];
                    son_n++;
                }

                if(son > heap[p])
                {
                    int tmp = heap[p];
                    heap[p] = son;
                    heap[son_n] = tmp;
                }
                else
                {
                    break;
                }

                p = son_n;
            }
        }

        public void BuildMaxHeap()
        {
            for(int s = H / 2; s >= 1; s--)
            {
                MaxHeapfy(s);
            }
        }

        public void Insert(int key)
        {
            H++;
            heap[H] = key;

            int son = H;

            while(son != 1)
            {
                int p = son / 2;
                if(heap[son] > heap[p])
                {
                    int tmp = heap[p];
                    heap[p] = heap[son];
                    heap[son] = tmp;
                    son /= 2;
                }
                else
                {
                    break;
                }
            }

        }

        public int ExtractMax()
        {
            int max = heap[1];
            heap[1] = heap[H];
            H--;

            MaxHeapfy(1);

            return max;
        }

        public int GetMax()
        {
            if (H == 0) return -1;
            return heap[1];
        }

    }


    
    class Program
    {

        static long Solve(int N, int M, PriorityQueue[] B)
        {
            long cnt = 0;

            //最終日(M日)からi日前
            for(int i = 1; i <= M; i++)
            {
                int max = -1;
                int selct_a = -1;

                for(int j = i; j >= 1; j--)
                {
                    int m = B[j].GetMax();
                    if(m > max)
                    {
                        max = m;
                        selct_a = j;
                    }
                }

                if (selct_a == -1) continue;

                cnt += max;
                B[selct_a].ExtractMax();
            }

            return cnt;
        }
        
        static void Main(string[] args)
        {
            string[] input = Console.ReadLine().Split(' ');
            int N = int.Parse(input[0]);
            int M = int.Parse(input[1]);

            PriorityQueue[] B = new PriorityQueue[M + 1];

            for (int i = 0; i <= M; i++) B[i] = new PriorityQueue();
 
            for(int i = 0; i < N; i++)
            {
                input = Console.ReadLine().Split(' ');
                int a = int.Parse(input[0]);
                int b = int.Parse(input[1]);
                if(a <= M) B[a].Insert(b); ;
            }

            Console.WriteLine(Solve(N, M, B));
            
        }
    }
}