USACO - Mixing Milk

Mixing Milk

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

Greedy - sorting

This problem is solved using a straight forward greedy algorithm, you can easily see that if you sort the input by prices and buy milk from those who sell milk cheaper, you will result in the optimum answer.
For example in the sample input you can see that : you need 100 gallons of milk and there are 5 farmers available for you to buy milk from. First you buy
10 gallons from the farmer who sells milk 3 cents per gallon, then
20 gallons from the farmer who sells milk 5 cents per gallon, then
30 gallons from the farmer who sells milk 6 cents per gallon, then
40 gallons from the farmer who sells milk 8 cents per gallon ( this farmer has 80 gallons available but you just need 40 gallons to reach 100 gallons each day )
So the answer is 10*3 + 20*5 + 30*6 + 40*8 = 30 + 100 + 180 + 320 = 630