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 PyPy3 (2.4.0) (judge verdict: Time Limit Exceeded)

def merge(a_list, b_list):
    res = []

    while len(a_list) > 0 and len(b_list) > 0:
        if a_list[0] > b_list[0]:
            res.append(a_list.pop(0))
        else:
            res.append((b_list.pop(0)))

    if len(a_list) > 0:
        res.extend(a_list)
    if len(b_list) > 0:
        res.extend(b_list)

    return res


N, M = map(int, input().split())
ab_list = [ list(map(int, input().split())) for i in range(N)]

score_list = [[] for i in range(M+1)]


for ab in ab_list:
    if ab[0] <= M:
        score_list[ab[0]].append(ab[1])
for i in range(len(score_list)):
    score_list[i].sort(reverse=True)

ans = 0
for i in range(0, M+1):
    if len(score_list[i]) == 0:
        continue
    ans += score_list[i].pop(0)
    if i < M and len(score_list[i]) > 0:
        score_list[i+1] = merge(score_list[i], score_list[i+1])

print(ans)