# [Usaco2005 Mar]Yogurt factory

### 题目描述

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.

### 输入格式

* 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.

• 第一行：两个整数 N 和 S，1 ≤ N ≤ 10000, 1 ≤ S ≤ 100
• 第二行到第 N + 1 行：第 i + 1 行有两个整数 Ci 和 Ai，1 ≤ Ci ≤ 5000, 1 ≤ Ai ≤ 10000

### 输出格式

* 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.

• 单个整数：表示生产酸奶的最小总费用

```4 5
88 200
89 400
97 300
91 500
```

### 样例输出

```126900

OUTPUT DETAILS:

700 吨酸奶，并存下 300 吨；第三个月不生产酸

Silver