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



Hint



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. 


 


 


题目意思：由于市场原因牛奶的生产价格会有所波动，要求在将来的n周里，该公司拥有一个足够大的仓库储存制好的牛奶，每个单位的牛奶每周储存

 所需的费用为s，现给出将来n周生产每单位牛奶的价格及客户所需的数量（即提前知道了下面几周的牛奶价格和客户需求！！！），求出该公司将来n周内所要支付的最小生产值。


 


解题思路：这是一道贪心题，贪心策略，每一个周只需要和下一个周对比，每一次更新下一个相邻的周，因为若下一个周被更新了，那么下一个周可以用来更新剩下的周，所以每一个周只需要对下一个周负责就可以了，可以看成一种带有传递性的比较。


 


上代码：


1 #include<stdio.h>
 2 struct Yogurt
 3 {
 4     int c;
 5     int y;
 6 };
 7 int main()
 8 {
 9     int n,s,i;
10     long long ans;
11     struct Yogurt a[10010];
12     a[0].c=999999;
13     while(scanf("%d%d",&n,&s)!=EOF)
14     {
15         for(i=1; i<=n; i++)
16         {
17             scanf("%d%d",&a[i].c,&a[i].y);
18         }
19         for(i=1; i<=n; i++)
20         {
21             if(a[i].c<a[i-1].c+s)
22                 a[i].c=a[i].c;
23             else
24                 a[i].c=a[i-1].c+s;
25         }
26         ans=0;
27         for(i=1; i<=n; i++)
28         {
29             ans=ans+a[i].c*a[i].y;
30         }
31         printf("%lld\n",ans);
32     }
33     return 0;
34 }