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 <iostream>
#include <algorithm>
#include <functional>
#include <vector>
#include <set>
#include <map>
#include <stack>
#include <queue>
#include <string>
#include <numeric>
#include <cmath>

#define rep(i, n) for (int i = 0; i < n; i++)
using namespace std;
typedef long long ll;
using P = pair<ll, ll>;

template<class T> inline bool chmin(T& a,T b) {if (a > b) {a = b; return true;} return false;}
template<class T> inline bool chmax(T& a,T b) {if (a < b) {a = b; return true;} return false;}

const ll MOD = 1000000007;
const int INF = 1<<30;
//const ll INF = (ll)1e18 + 1;

int main(){
  int N, M; cin >> N >> M;
  vector<vector<int>> jobs(M);
  rep(i, N){
    int a, b; cin >> a >> b;
    if (a > M) continue;
    jobs[M - a].push_back(b);
  }
  priority_queue<int, vector<int>> q;
  ll ans = 0;
  for (int i = M - 1; i >= 0; --i){
    for (int b: jobs[i]){
      q.push(b);
    }
    if (!q.empty()){
      ans += q.top();
      q.pop();
    }
  }
  cout << ans << endl;
}