Vanya and Exams

Venya 有 $n$ 门课程，每门课程他最多获得 $r$ 学分，他只要所有课程的平均学分大于等于 $avg$，他就可以获得奖学金。 目前对于第 $i$ 门课程，他已经获得了 $a_i$ 学分，剩下的每个学分，都需要写 $b_i$ 篇论文才能得到。请问 Venya 最少要写多少论文才能获得奖学金？ 数据保证 $1\le n\le 10^5,1\le avg\le 10^6,1\le r\le 10^9$。

Vanya wants to pass $ n $ exams and get the academic scholarship. He will get the scholarship if the average grade mark for all the exams is at least $ avg $ . The exam grade cannot exceed $ r $ . Vanya has passed the exams and got grade $ a_{i} $ for the $ i $ -th exam. To increase the grade for the $ i $ -th exam by 1 point, Vanya must write $ b_{i} $ essays. He can raise the exam grade multiple times. What is the minimum number of essays that Vanya needs to write to get scholarship?

The first line contains three integers n, r, avg (1 ≤ n ≤ 105, 1 ≤ r ≤ 109, 1 ≤ avg ≤ min(r, 106)) — the number of exams, the maximum grade and the required grade point average, respectively. Each of the following n lines contains space-separated integers ai and bi (1 ≤ ai ≤ r, 1 ≤ bi ≤ 106).

In the first line print the minimum number of essays.

5 5 4
5 2
4 7
3 1
3 2
2 5

4

2 5 4
5 2
5 2

0

In the first sample Vanya can write 2 essays for the 3rd exam to raise his grade by 2 points and 2 essays for the 4th exam to raise his grade by 1 point. In the second sample, Vanya doesn't need to write any essays as his general point average already is above average.