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 Python (3.4.3) (judge verdict: Accepted)

MOD = 10 ** 9 + 7
INF = 10 ** 11
import sys
sys.setrecursionlimit(100000000)
from heapq import heapify,heappop,heappush

def main():
    N,M = map(int,input().split())
    works = [tuple(map(int,input().split())) for _ in range(N)]
    works.sort(key = lambda x:x[0])
    q = []
    ans = 0
    j = 0
    for i in range(M):
        while j < N and works[j][0] <= i + 1:
            heappush(q,-works[j][1])
            j += 1
        if q:
            p = heappop(q)
            ans -= p
    print(ans)
if __name__ == '__main__':
    main()