bzoj1680[Usaco2005 Mar]Yogurt factory

 

Description

The cows have purchased a yogurt factory that makes world-famous Yucky Yogurt. Over the next N (1 <= N <= 10,000) weeks, the price of milk and labor will fluctuate weekly such that it will cost the company C_i (1 <= C_i <= 5,000) cents to produce one unit of yogurt in week i. Yucky's factory, being well-designed, can produce arbitrarily many units of yogurt each week. Yucky Yogurt owns a warehouse that can store unused yogurt at a constant fee of S (1 <= S <= 100) cents per unit of yogurt per week. Fortuitously, yogurt does not spoil. Yucky Yogurt's warehouse is enormous, so it can hold arbitrarily many units of yogurt. Yucky wants to find a way to make weekly deliveries of Y_i (0 <= Y_i <= 10,000) units of yogurt to its clientele (Y_i is the delivery quantity in week i). Help Yucky minimize its costs over the entire N-week period. Yogurt produced in week i, as well as any yogurt already in storage, can be used to meet Yucky's demand for that week.

Input

* Line 1: Two space-separated integers, N and S. * Lines 2..N+1: Line i+1 contains two space-separated integers: C_i and Y_i.

Output

* Line 1: Line 1 contains a single integer: the minimum total cost to satisfy the yogurt schedule. Note that the total might be too large for a 32-bit integer.

Sample Input

4 5
88 200
89 400
97 300
91 500

Sample Output

126900

OUTPUT DETAILS:

In week 1, produce 200 units of yogurt and deliver all of it. In
week 2, produce 700 units: deliver 400 units while storing 300
units. In week 3, deliver the 300 units that were stored. In week
4, produce and deliver 500 units.

 

最恨纯英文题……

题意是有n天，第i天生产的费用是c[i]，要生产y[i]个产品，可以用当天的也可以用以前的。每单位产品保存一天的费用是s。求最小费用

贪心……对于每一天可以从当天转移也可以从以前转移，只要保存一个以前的最小值，在这一天加一个s再跟c比较，更新完最小值直接更新答案。

#include<cstdio>
inline int read()
{
    int x=0,f=1;char ch=getchar();
    while(ch<'0'||ch>'9'){if(ch=='-')f=-1;ch=getchar();}
    while(ch>='0'&&ch<='9'){x=x*10+ch-'0';ch=getchar();}
    return x*f;
}
int n,s,save=100000000;
long long ans;
int main()
{
	n=read();
	s=read();
	for (int i=1;i<=n;i++)
	 {
	 	int w=read(),c=read();
	 	save+=s;
	 	if (save>w)save=w;
	 	ans+=(long long)save*c;
	 }
	 printf("%lld",ans);
}