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;
using System.Text;
using System.Threading.Tasks;

namespace ABC137_D
{
    class Program
    {
        static void Main(string[] args)
        {
            var NM = Array.ConvertAll(Console.ReadLine().Split(), int.Parse);
            var N = NM[0];
            var M = NM[1];

            var list = new List<int>[M + 1];
            for (int i = 0; i < M + 1; i++)
            {
                list[i] = new List<int>();
            }

            for (int i = 0; i < N; i++)
            {
                var line = Array.ConvertAll(Console.ReadLine().Split(), int.Parse);
                if (line[0] > M) continue;
                list[line[0]].Add(line[1]);
            }

            long ans = 0;
            var cand = new List<int>();
            for (int i = 0; i < M; i++)
            {
                cand.AddRange(list[i + 1]);
                cand.Sort();

                if (cand.Count == 0) continue;

                var mx = cand[cand.Count - 1];
                ans += mx;
                cand.RemoveAt(cand.Count - 1);
            }

            Console.WriteLine(ans);
        }
    }
}