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;
#define endl "\n"
#define watch(x) cout << #x << " is " << x << endl;
#define print(a, l, r)          \
	for (int i = l; i < r; i++) \
		cout << a[i] << (i == (r - 1) ? endl : " ");
const long long M = 1000000007;
int main()
{

	ios_base::sync_with_stdio(false);
	cin.tie(NULL);
	cout.tie(NULL);

	long long n, m;
	cin >> n >> m;

	map<int, vector<int>> mp;
	for (int i = 0; i < n; i++)
	{
		int a, b;
		cin >> a >> b;
		mp[a].push_back(b);
	}
	long long ans = 0;
	priority_queue<int> que;
	for (int i = 1; i <= m; i++)
	{
		for (auto j : mp[i])
		{
			que.push(j);
		}
		if (!que.empty())
		{
			ans += que.top();
			/* 	cout << (que.top()) << endl; */
			que.pop();
		}
	}
	cout << ans << endl;

	return 0;
}