Burglar and Matches
机翻
一个窃贼进入了一个火柴仓库，想尽可能多地偷取火柴。仓库里有 m个集装箱，在第 i 个集装箱里有 ai个火柴盒，每个火柴盒里有 bi火柴。所有的火柴盒都是一样大的。窃贼的背包可以容纳 n火柴盒。你的任务是找出窃贼可以带走的最大数量的火柴。他没有时间重新排列火柴盒中的火柴，这就是为什么他只选择了不超过 n火柴盒，使其中的火柴总量达到最大。
输入的第一行包含整数 n(1 ≤ n ≤ 2·108)和整数 m(1 ≤ m ≤ 20).第 i + 1-第1行包含一对数字 ai和 bi(1 ≤ ai ≤ 108, 1 ≤ bi ≤ 10).所有的输入数字都是整数。
输出唯一的数字--问题的答案。
Description
A burglar got into a matches warehouse and wants to steal as many matches as possible. In the warehouse there are m containers, in the i-th container there are ai matchboxes, and each matchbox contains bi matches. All the matchboxes are of the same size. The burglar's rucksack can hold n matchboxes exactly. Your task is to find out the maximum amount of matches that a burglar can carry away. He has no time to rearrange matches in the matchboxes, that's why he just chooses not more than n matchboxes so that the total amount of matches in them is maximal.
The first line of the input contains integer n (1 ≤ n ≤ 2·108) and integer m (1 ≤ m ≤ 20). The i + 1-th line contains a pair of numbers ai and bi (1 ≤ ai ≤ 108, 1 ≤ bi ≤ 10). All the input numbers are integer.
Output the only number — answer to the problem.
Input
The first line of the input contains integer n (1 ≤ n ≤ 2·108) and integer m (1 ≤ m ≤ 20). The i + 1-th line contains a pair of numbers ai and bi (1 ≤ ai ≤ 108, 1 ≤ bi ≤ 10). All the input numbers are integer.
Output
Output the only number — answer to the problem.
Samples
7 3
5 10
2 5
3 6
62
3 3
1 3
2 2
3 1
7