256. Balloons

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 Ai balloons during one minute and then he or she must have at least Bi minutes of rest.
You are to write a program that computes minimal possible time of filling up M balloons.

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

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

Sample test(s)

Input
10 3
4 4
3 4
2 3

Output
5
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Author:Andrew V. Lazarev
Resource:Saratov SU Contest: Golden Fall 2004
Date:October 2, 2004











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