## 21378: Greedy Mouse

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## 题目描述

A fat mouse prepared M pounds of cat food, ready to trade with the cats guarding the warehouse containing his favorite food:peanut. The warehouse has N rooms. The ith room containsW[i] pounds of peanut and requires F[i] pounds of cat food.  Fatmouse does not have to trade for all the peanut in the room, instead, he may get W[i]*a% pounds of peanut if he pays F[i]*a% pounds of cat food. The mouse is a greedy mouse, so can you tell him the maximum amount of peanut he can obtain.

## 输入

The input consists of multiple test cases.  Each test case begins with a line containing two non-negative integers M and N. Then N lines follow,  each contains two non-negative integers W[i] and F[i] respectively. The test case is terminated by two -1.  All integers are not greater than 1000.

## 输出

For each test case, print in a single line a real number accurate up to 3 decimal places, which is the maximum amount of penaut that FatMouse can obtain.

## 样例输入

5 3
7 2
4 3
5 2
20 3
25 18
24 15
15 10
-1 -1

## 样例输出

13.333
31.500

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