Burglar and Matches

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 a_i matchboxes, and each matchbox contains b_i 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 a_i and b_i (1 ≤ a_i ≤ 10⁸, 1 ≤ b_i ≤ 10). All the input numbers are integer.

Output the only number — answer to the problem.

7 3
5 10
2 5
3 6

62

3 3
1 3
2 2
3 1

7