H. 混合牛奶

牛奶包装是一个如此低利润的生意,所以尽可能低的控制初级产品(牛奶)的价格变的十分重要。 请帮助快乐的牛奶制造者(Merry Milk Makers)以可能的最廉价的方式取得他们所需的牛奶。 快乐的牛奶制造公司从一些农民那购买牛奶，每个农民卖给牛奶制造公司的价格不一定相同。 而且,如一只母牛一天只能生产一定量的牛奶,农民每一天只有一定量的牛奶可以卖。 每天,快乐的牛奶制造者从每个农民那购买一定量的牛奶,少于或等于农民所能提供的最大值。 给出快乐牛奶制造者的每日的牛奶需求,连同每个农民的可提供的牛奶量和每加仑的价格,请计算快乐的牛奶制造者所要付出钱的最小值。 注意: 每天农民生产的牛奶的总数对快乐的牛奶制造者来说足够的。

第 1 行:二个整数, N 和 M。 第一个数值,N,(0<= N<=2,000,000)是快乐的牛奶制造者的一天需要牛奶的数量。 第二个数值,M,(0<= M<=5,000)是他们可能从农民那买到的数目。 第 2 到 M+1 行:每行二个整数,Pi 和 Ai。 Pi(0<= Pi<=1,000) 是农民 i 牛奶的价格。 Ai(0 <= Ai <= 2,000,000)是农民 i 一天能卖给快乐的牛奶制造者的牛奶数量。

单独的一行包含单独的一个整数，表示快乐的牛奶制造者拿到所需的牛奶所要的最小费用

Since milk packaging is such a low margin business, it is important to keep the price of the raw product (milk) as low as possible. Help Merry Milk Makers get the milk they need in the cheapest possible manner.

The Merry Milk Makers company has several farmers from which they may buy milk, and each one has a (potentially) different price at which they sell to the milk packing plant. Moreover, as a cow can only produce so much milk a day, the farmers only have so much milk to sell per day. Each day, Merry Milk Makers can purchase an integral amount of milk from each farmer, less than or equal to the farmer's limit.

Given the Merry Milk Makers' daily requirement of milk, along with the cost per gallon and amount of available milk for each farmer, calculate the minimum amount of money that it takes to fulfill the Merry Milk Makers' requirements.

Note: The total milk produced per day by the farmers will be sufficient to meet the demands of the Merry Milk Makers.

PROGRAM NAME: milk INPUT FORMAT Line 1: Two integers, N and M. The first value, N, (0 <= N <= 2,000,000) is the amount of milk that Merry Milk Makers' want per day. The second, M, (0 <= M <= 5,000) is the number of farmers that they may buy from.

Lines 2 through M+1: The next M lines each contain two integers, Pi and Ai. Pi (0 <= Pi <= 1,000) is price in cents that farmer i charges. Ai (0 <= Ai <= 2,000,000) is the amount of milk that farmer i can sell to Merry Milk Makers per day.

SAMPLE INPUT (file milk.in) 100 5 5 20 9 40 3 10 8 80 6 30

OUTPUT FORMAT A single line with a single integer that is the minimum price that Merry Milk Makers can get their milk at for one day.

SAMPLE OUTPUT (file milk.out) 630