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# (.NET Core 3.1.201) (judge verdict: Time Limit Exceeded)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ABC137D
{
    class Program
    {
        static void Main(string[] args)
        {
            string[] NM = Console.ReadLine().Split(' ');
            int N = int.Parse(NM[0]);
            int M = int.Parse(NM[1]);
            int[] A = new int[N];
            int[] B = new int[N];
            for(int i = 0; i < N; i++)
            {
                string[] AB = Console.ReadLine().Split(' ');
                A[i] = int.Parse(AB[0]);
                B[i] = int.Parse(AB[1]);
            }
            var housyu = new List<int>[100001];
            for(int i = 0; i < 100001; i++)
            {
                housyu[i] = new List<int>();
            }
            for(int i = 0; i < N; i++)
            {
                housyu[A[i]].Add(B[i]);
            }
            long ans = 0;
            for(int i = 1; i <= M; i++)
            {
                int max = 0;
                int[] maxat=new int[2];
                for(int j = 1; j <= i; j++)
                {
                    int l = housyu[j].Count;
                    for(int k = 0; k < l; k++)
                    {
                        if (housyu[j].ElementAt(k) >= max)
                        {
                            max = housyu[j].ElementAt(k);
                            maxat[0] = j;
                            maxat[1] = k;
                        }
                    }
                }
                //Console.WriteLine(string.Join(" ", maxat));
                ans += max;
                if (max > 0)
                {
                    housyu[maxat[0]].RemoveAt(maxat[1]);
                }
            }
            Console.WriteLine(ans);
        }
    }
}