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.

Input

The first line contains 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.

The next M lines (Line 2 through M+1) 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. 

Output

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 Input

100 5 5 20 9 40 3 10 8 80 6 30

Sample Output

630

/*This Code is Submitted by 黄仪标 for Problem 1003 at 2012-11-05 22:14:13*/#include <iostream>using namespace std;template<typename T>void Swap(T &lps, T &rps){    T tmp = lps;    lps = rps;    rps = tmp;}void SortASC(int *p, long *a, int size){    int itmp;    long ltmp;    for (int i = 0; i < size; i++)    {        for (int j = i+1; j < size; j++)        {            if (p[i] > p[j])            {                Swap(p[i], p[j]);//这里可以调用STL里面的swap                Swap(a[i], a[j]);//不过这里我是自己写的一个模板            }        }    }}int main(){    long milkRequired = 0, //公司的牛奶需求量         numOfFarmer  = 0; //能够提供牛奶的农夫数量    int i;    cin >> milkRequired >> numOfFarmer;    int *perPrice = new int[numOfFarmer];   //农夫i售出的牛奶的单价    long *numOfMilk = new long[numOfFarmer];//农夫i可以售的牛奶的数量    for (i = 0; i < numOfFarmer; i++)    {        //读入单价及对应的数量        cin >> perPrice[i] >> numOfMilk[i];    }    //将单价从小到大排序    SortASC(perPrice, numOfMilk, numOfFarmer);    i = 0;    long minCost = 0,         tmp;    while (milkRequired != 0 && i < numOfFarmer)    {        if (milkRequired >= numOfMilk[i])        {            minCost += numOfMilk[i] * perPrice[i];            milkRequired -= numOfMilk[i];            //cout << "mincost: " << minCost << endl;        }        else        {            minCost += milkRequired * perPrice[i];            milkRequired = 0;           // cout << "mincost: " << minCost << endl;        }        i ++;    }    cout << minCost << endl;    delete []perPrice;    delete []numOfMilk;    return 0;}