CF19B.Checkout Assistant

Bob came to a cash & carry store, put n items into his trolley, and went to the checkout counter to pay. Each item is described by its price c__i and time t__i in seconds that a checkout assistant spends on this item. While the checkout assistant is occupied with some item, Bob can steal some other items from his trolley. To steal one item Bob needs exactly 1 second. What is the minimum amount of money that Bob will have to pay to the checkout assistant? Remember, please, that it is Bob, who determines the order of items for the checkout assistant.

鲍勃来到一家自选商店，将 $n$ 件商品放入购物车，然后前往收银台付款。每件商品由其价格 $c_i$ 和收银员处理该商品所需的时间 $t_i$（单位：秒）描述。当收银员正在处理某件商品时，鲍勃可以趁机从自己的购物车中偷走其他商品；偷走一件商品恰好需要 1 秒。鲍勃最少需要向收银员支付多少钱？请注意，鲍勃可以自行决定收银员处理商品的顺序。

The first input line contains number n (1 ≤ n ≤ 2000). In each of the following n lines each item is described by a pair of numbers t__i, c__i (0 ≤ t__i ≤ 2000, 1 ≤ c__i ≤ 109). If t__i is 0, Bob won't be able to steal anything, while the checkout assistant is occupied with item i.

第一行输入包含一个数字 $n$（$1 \leq n \leq 2000$）。接下来的 $n$ 行中，每行用一对数字 $t_i$、$c_i$（$0 \leq t_i \leq 2000$，$1 \leq c_i \leq 10^9$）描述一件商品。若 $t_i = 0$，则在收银员处理第 $i$ 件商品期间，Bob 将无法窃取任何物品。

Output one number — answer to the problem: what is the minimum amount of money that Bob will have to pay.

输出一个数字——该问题的答案：鲍勃需要支付的最少金额。