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 \le n \le 2 \cdot 108) and integer m (1 \le m \le 20). The i+1-th line contains a pair of numbers ai and bi (1 \le ai \le 108,1 \le bi \le 10). All the input numbers are integer.

Output the only number - answer to the problem.