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: Accepted)

using System;
using System.Collections.Generic;
using System.Linq;
using static System.Console;
using static System.Math;

class MainClass : Scanner
{
    static void Main()
    {
        var (N, M) = ReadStream<int, int>();
        var ab = new (int a, long b)[N];
        for (int i = 0; i < N; i++)
        {
            ab[i] = ReadStream<int, long>();
        }
        Array.Sort(ab, (x, y) => x.a.CompareTo(y.a));
        var pque = new PriorityQueue<(int a, long b)>(N, (x, y) => -x.b.CompareTo(y.b));
        int idx = 0;
        long ans = 0;
        for (int i = 1; i <= M; i++)
        {
            while (idx<N&&ab[idx].a <= i) pque.Push(ab[idx++]);
            if (pque.Size == 0) continue;
            ans += pque.Pop().b;
        }
        WriteLine(ans);
    }
}
public class PriorityQueue<T>
{
    public long Size { get; private set; } = 0;
    public long MaxSize { get; private set; } = 0;
    public T[] m_heap;
    private Comparison<T> Comp = null;

    public PriorityQueue(long maxSize, Comparison<T> comp)
    {
        if (maxSize <= 0)
            throw new Exception();
        MaxSize = maxSize;
        m_heap = new T[maxSize];

        Comp = comp;
    }

    public void Push(T x)
    {
        if (Size == MaxSize)
        {
            T[] new_heap = new T[MaxSize << 1];
            Array.Copy(m_heap, new_heap, MaxSize);
            m_heap = new_heap;
            MaxSize <<= 1;
        }

        long i = Size++;
        while (i > 0)
        {
            long p = (i - 1) / 2;
            if (Comp(m_heap[p], x) <= 0)
                break;
            m_heap[i] = m_heap[p];
            i = p;
        }
        m_heap[i] = x;
    }

    public T Pop()
    {
        if (Size == 0)
            throw new Exception("Queue is empty.");

        T result = m_heap[0];
        T x = m_heap[--Size];
        long i = 0;
        while (i * 2 + 1 < Size)
        {
            long c1 = i * 2 + 1, c2 = i * 2 + 2;
            if (c2 < Size && Comp(m_heap[c2], m_heap[c1]) < 0)
                c1 = c2;
            if (Comp(m_heap[c1], x) >= 0)
                break;
            m_heap[i] = m_heap[c1];
            i = c1;
        }
        m_heap[i] = x;

        return result;
    }
}
class Scanner
{
    public static int RInt() => int.Parse(Console.ReadLine());
    public static long RLong() => long.Parse(Console.ReadLine());
    public static string RString() => Console.ReadLine();
    public static double RDouble() => double.Parse(Console.ReadLine());

    public static int[] RInts() => Array.ConvertAll(Console.ReadLine().Split(), int.Parse);
    public static int[] RInts(Func<string, int> func) => Console.ReadLine().Split().Select(func).ToArray();
    public static long[] RLongs() => Array.ConvertAll(Console.ReadLine().Split(), long.Parse);
    public static long[] RLongs(Func<string, long> func) => Console.ReadLine().Split().Select(func).ToArray();
    public static double[] RDoubles() => Array.ConvertAll(Console.ReadLine().Split(), double.Parse);
    public static double[] RDoubles(Func<string, double> func) => Console.ReadLine().Split().Select(func).ToArray();
    public static string[] RStrings() => Console.ReadLine().Split();
    //public static string[] RStrings(Func<string, string> func) => Console.ReadLine().Split().Select(func).ToArray();
    public static T1 ReadStream<T1>()
    {
        var r = RStrings();
        var r1 = (T1)Convert.ChangeType(r[0], typeof(T1));
        return r1;
    }
    public static (T1, T2) ReadStream<T1, T2>()
    {
        var r = RStrings();
        var r1 = (T1)Convert.ChangeType(r[0], typeof(T1));
        var r2 = (T2)Convert.ChangeType(r[1], typeof(T2));
        return (r1, r2);
    }
    public static (T1, T2, T3) ReadStream<T1, T2, T3>()
    {
        var r = RStrings();
        var r1 = (T1)Convert.ChangeType(r[0], typeof(T1));
        var r2 = (T2)Convert.ChangeType(r[1], typeof(T2));
        var r3 = (T3)Convert.ChangeType(r[2], typeof(T3));
        return (r1, r2, r3);
    }
    public static (T1, T2, T3, T4) ReadStream<T1, T2, T3, T4>()
    {
        var r = RStrings();
        var r1 = (T1)Convert.ChangeType(r[0], typeof(T1));
        var r2 = (T2)Convert.ChangeType(r[1], typeof(T2));
        var r3 = (T3)Convert.ChangeType(r[2], typeof(T3));
        var r4 = (T4)Convert.ChangeType(r[3], typeof(T4));
        return (r1, r2, r3, r4);
    }
}