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.Text;
using System.Collections.Generic;
using System.Collections;
using System.Linq; 
using E = System.Linq.Enumerable; 


class Program {
    static void Main(string[] args) {
        
        var NM = Console.ReadLine().Split(' ').Select(x => int.Parse(x)).ToArray();
        var N = NM[0];
        var M = NM[1];
        var inp = new List<int>[100001];
        
        for(int i = 1;i <= 100000;i++){
            inp[i] = new List<int>();
        }
        for(int i = 0;i < N;i++){
            var AB = Console.ReadLine().Split(' ').Select(x => int.Parse(x)).ToArray();
            
            var A = AB[0];
            var B = AB[1];
           // Console.WriteLine(A);
            inp[A].Add(B);
        }
        var pq = new PriorityQueue<long>();
        long ans = 0;
        for(int i = 1;i <= M;i++){
            foreach(var item in inp[i]){
                pq.Enqueue(item);
            }
            if(pq.Count >= 1){
                ans += pq.Dequeue();
            }
            
        }
        Console.WriteLine(ans);
        
     }
}
     
     
                   
public class PriorityQueue<T> : IEnumerable<T> { private readonly List<T> _data = new List<T>(); private readonly IComparer<T> _comparer; private readonly bool _isDescending; public PriorityQueue(IComparer<T> comparer, bool isDescending = true) { _comparer = comparer; _isDescending = isDescending; } public PriorityQueue(Comparison<T> comparison, bool isDescending = true) : this(Comparer<T>.Create(comparison), isDescending) { } public PriorityQueue(bool isDescending = true) : this(Comparer<T>.Default, isDescending) { } public void Enqueue(T item) { _data.Add(item); var childIndex = _data.Count - 1; while (childIndex > 0) { var parentIndex = (childIndex - 1) / 2; if (Compare(_data[childIndex], _data[parentIndex]) >= 0) break; Swap(childIndex, parentIndex); childIndex = parentIndex; } } public T Dequeue() { var lastIndex = _data.Count - 1; var firstItem = _data[0]; _data[0] = _data[lastIndex]; _data.RemoveAt(lastIndex--); var parentIndex = 0; while (true) { var childIndex = parentIndex * 2 + 1; if (childIndex > lastIndex) break; var rightChild = childIndex + 1; if (rightChild <= lastIndex && Compare(_data[rightChild], _data[childIndex]) < 0) childIndex = rightChild; if (Compare(_data[parentIndex], _data[childIndex]) <= 0) break; Swap(parentIndex, childIndex); parentIndex = childIndex; } return firstItem; } public T Peek() { return _data[0]; } private void Swap(int a, int b) { var tmp = _data[a]; _data[a] = _data[b]; _data[b] = tmp; } private int Compare(T a, T b) { return _isDescending ? _comparer.Compare(b, a) : _comparer.Compare(a, b); } public int Count => _data.Count; public IEnumerator<T> GetEnumerator() { return _data.GetEnumerator(); } IEnumerator IEnumerable.GetEnumerator() => GetEnumerator(); }