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++14 (GCC 5.4.1) (judge verdict: Accepted)

#include<bits/stdc++.h>
using namespace std;
int main(){
    int N,M;
    cin>>N>>M;
    vector<pair<int,int>> vp;
    for(int i=0;i<N;i++){
        int x,y;
        cin>>x>>y;
        vp.push_back(make_pair(x,y));
    }
    sort(vp.begin(),vp.end());
    long long ans=0;
    int next=0;
    priority_queue<int> pq;
    for(int i=0;i<=M;i++){
        while(next<N&&vp[next].first==i){
            pq.push(vp[next].second);
            next++;
        }
        if(!pq.empty()){
            ans += pq.top();
            pq.pop();
        }
    }
    cout<<ans<<endl;

}