time limit per test: 0.25 sec.
memory limit per test: 65536 KB

input: standard
output: standard

Due to the preparation to the wedding you have to fill up M balloons. There are N volunteers and one device for balloons filling up. Device can fill at most one balloon at a time. Each volunteer can continuously operate the device for 1 minute.
Volunteer number i can fill A_i balloons during one minute and then he or she must have at least B_i minutes of rest.
You are to write a program that computes minimal possible time of filling up M balloons.

There are two integers M and N on the first line (1 <= M <= 100, 1 <= N <= 10).
Next N lines contain two integers each: A_i and B_i (0 <= A_i <= 10, 1 <= B_i <= 4).

Print one integer T - the minimal number of minutes required to filling M balloons.

Input

10 3
4 4
3 4
2 3