CF116A.Tram

Linear Kingdom has exactly one tram line. It has n stops, numbered from 1 to n in the order of tram's movement. At the i-th stop a__i passengers exit the tram, while b__i passengers enter it. The tram is empty before it arrives at the first stop. Also, when the tram arrives at the last stop, all passengers exit so that it becomes empty.

Your task is to calculate the tram's minimum capacity such that the number of people inside the tram at any time never exceeds this capacity. Note that at each stop all exiting passengers exit before any entering passenger enters the tram.

线性王国只有一条有轨电车线路。该线路共有 $n$ 个车站，按电车行驶方向编号为 $1$ 至 $n$。在第 $i$ 个车站，有 $a_i$ 名乘客下车，同时有 $b_i$ 名乘客上车。电车在到达第一个车站前为空车；此外，当电车抵达最后一个车站时，所有乘客均下车，使电车再次变为空车。

你的任务是计算电车所需的最小容量，使得电车在任意时刻的载客人数均不超过该容量。注意：在每个车站，所有下车乘客先全部下车，然后上车乘客才开始上车。

The first line contains a single number n (2 ≤ n ≤ 1000) — the number of the tram's stops.

Then n lines follow, each contains two integers a__i and b__i (0 ≤ a__i, b__i ≤ 1000) — the number of passengers that exits the tram at the i-th stop, and the number of passengers that enter the tram at the i-th stop. The stops are given from the first to the last stop in the order of tram's movement.

• The number of people who exit at a given stop does not exceed the total number of people in the tram immediately before it arrives at the stop. More formally, . This particularly means that _a_1 = 0.
• At the last stop, all the passengers exit the tram and it becomes empty. More formally, .
• No passenger will enter the train at the last stop. That is, b__n = 0.

第一行包含一个整数 $ n $（$ 2 \leq n \leq 1000 $）—— 表示有轨电车的停靠站数量。

接下来有 $ n $ 行，每行包含两个整数 $ a_i $ 和 $ b_i $（$ 0 \leq a_i, b_i \leq 1000 $）—— 分别表示在第 $ i $ 个停靠站下车的乘客数量和上车的乘客数量。这些停靠站按有轨电车运行顺序从第一站到最后一站依次给出。

• 在某一停靠站下车的乘客数量不超过该站到达前车厢内的总人数。更准确地说，满足 。特别地，这意味着 $ a_1 = 0 $。
• 在最后一站，所有乘客均下车，车厢变为空。更准确地说，满足 。
• 在最后一站没有乘客上车，即 $ b_n = 0 $。

Print a single integer denoting the minimum possible capacity of the tram (0 is allowed).

输出一个整数，表示电车的最小可能容量（允许为 0）。

For the first example, a capacity of 6 is sufficient:

• At the first stop, the number of passengers inside the tram before arriving is 0. Then, 3 passengers enter the tram, and the number of passengers inside the tram becomes 3.
• At the second stop, 2 passengers exit the tram (1 passenger remains inside). Then, 5 passengers enter the tram. There are 6 passengers inside the tram now.
• At the third stop, 4 passengers exit the tram (2 passengers remain inside). Then, 2 passengers enter the tram. There are 4 passengers inside the tram now.
• Finally, all the remaining passengers inside the tram exit the tram at the last stop. There are no passenger inside the tram now, which is in line with the constraints.

Since the number of passengers inside the tram never exceeds 6, a capacity of 6 is sufficient. Furthermore it is not possible for the tram to have a capacity less than 6. Hence, 6 is the correct answer.

对于第一个样例，容量为 6 即可满足要求：

• 在第一站，电车到达前车内乘客数为 0。然后有 3 名乘客上车，此时车内乘客数变为 3。
• 在第二站，有 2 名乘客下车（车内剩余 1 名乘客），然后有 5 名乘客上车，此时车内乘客数为 6。
• 在第三站，有 4 名乘客下车（车内剩余 2 名乘客），然后有 2 名乘客上车，此时车内乘客数为 4。
• 最后，在终点站，所有剩余乘客均下车，此时车内乘客数为 0，符合题设约束。

由于电车内的乘客数从未超过 6，因此容量为 6 是足够的。此外，电车的容量不可能小于 6。故正确答案为 6。