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

            long ans = 0;
            var cand = new PriorityQueue<int>(true);
            for (int i = 0; i < M; i++)
            {
                if (list[i + 1] != null)
                    foreach (var item in list[i + 1])
                    {
                        cand.Push(item);
                    }

                if (cand.Count == 0) continue;
                ans += cand.Pop();
            }

            Console.WriteLine(ans);
        }
    }

    class PriorityQueue<T> where T : IComparable<T>
    {
        public int Count { get; private set; }
        private bool Descendance;
        private T[] data = new T[65536];
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public PriorityQueue(bool descendance = false) { Descendance = descendance; }
        public T Top
        {
            [MethodImpl(MethodImplOptions.AggressiveInlining)]
            get { ValidateNonEmpty(); return data[1]; }
        }
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public T Pop()
        {
            var top = Top;
            var elem = data[Count--];
            int index = 1;
            while (true)
            {
                if ((index << 1) >= Count)
                {
                    if (index << 1 > Count) break;
                    if (elem.CompareTo(data[index << 1]) > 0 ^ Descendance) data[index] = data[index <<= 1];
                    else break;
                }
                else
                {
                    var nextIndex = data[index << 1].CompareTo(data[(index << 1) + 1]) <= 0 ^ Descendance ? (index << 1) : (index << 1) + 1;
                    if (elem.CompareTo(data[nextIndex]) > 0 ^ Descendance) data[index] = data[index = nextIndex];
                    else break;
                }
            }
            data[index] = elem;
            return top;
        }
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public void Push(T value)
        {
            int index = ++Count;
            if (data.Length == Count) Extend(data.Length * 2);
            while ((index >> 1) != 0)
            {
                if (data[index >> 1].CompareTo(value) > 0 ^ Descendance) data[index] = data[index >>= 1];
                else break;
            }
            data[index] = value;
        }
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        private void Extend(int newSize)
        {
            T[] newDatas = new T[newSize];
            data.CopyTo(newDatas, 0);
            data = newDatas;
        }
        private void ValidateNonEmpty() { if (Count == 0) throw new Exception(); }
    }
}