Go to movies

Winter holiday is coming!As the monitor, LeLe plans to go to the movies.
Because the winter holiday tickets are pretty expensive, LeLe decideds to try group-buying.

There are multiple test cases, about $20$ cases. The first line of input contains two integers $n,m(1 \leq n,m \leq 100)$. $n$ indicates the number of the students. $m$ indicates how many cinemas have offered group-buying.

For the $m$ lines,each line contains two integers $a_i,b_i(1 \leq ai,bi \leq 100)$, indicating the choices of the group buying cinemas offered which means you can use $b_i$ yuan to buy $a_i$ tickets in this cinema.

For each case, please help LeLe **choose a cinema** which costs the least money. Output the total money LeLe should pay.

3 2
2 2
3 5

4

LeLe can buy four tickets with four yuan in cinema 1.

BestCoder Round #34