Saitama Destroys Hotel

题意翻译

# 题面 埼玉老师又意外地摧毁了一家酒店。为了赔偿酒店公司，杰诺斯自愿在其另一家酒店经营电梯。电梯很特别，它从顶层开始，只能向下移动，并且有无限的容量。楼层编号从0到S，电梯开始于S层，时间为0。 电梯只需1秒钟就可以向下移动1层楼，接送乘客的时间可以忽略不计。杰诺斯有一份详细的旅客到达时间和地点的清单。请确定杰诺斯将所有乘客带到0楼需要多长时间。 # 输入 n和s(1<=n<=100)(1<=s<=1000) 即 乘客人数 和 题面中的S（以秒为单位） 接下来n行包含两个数字，n个乘客的楼层 和 到达时间 # 输出 将所有乘客带到0楼需要的时间。

题目描述

Saitama accidentally destroyed a hotel again. To repay the hotel company, Genos has volunteered to operate an elevator in one of its other hotels. The elevator is special — it starts on the top floor, can only move down, and has infinite capacity. Floors are numbered from $ 0 $ to $ s $ and elevator initially starts on floor $ s $ at time $ 0 $ . The elevator takes exactly $ 1 $ second to move down exactly $ 1 $ floor and negligible time to pick up passengers. Genos is given a list detailing when and on which floor passengers arrive. Please determine how long in seconds it will take Genos to bring all passengers to floor $ 0 $ .

输入输出格式

输入格式

The first line of input contains two integers $ n $ and $ s $ ( $ 1<=n<=100 $ , $ 1<=s<=1000 $ ) — the number of passengers and the number of the top floor respectively. The next $ n $ lines each contain two space-separated integers $ f_{i} $ and $ t_{i} $ ( $ 1<=f_{i}<=s $ , $ 1<=t_{i}<=1000 $ ) — the floor and the time of arrival in seconds for the passenger number $ i $ .

输出格式

Print a single integer — the minimum amount of time in seconds needed to bring all the passengers to floor $ 0 $ .

输入输出样例

输入样例 #1

3 7
2 1
3 8
5 2

输出样例 #1

11

输入样例 #2

5 10
2 77
3 33
8 21
9 12
10 64

输出样例 #2

79

说明

In the first sample, it takes at least $ 11 $ seconds to bring all passengers to floor $ 0 $ . Here is how this could be done: 1\. Move to floor $ 5 $ : takes $ 2 $ seconds. 2\. Pick up passenger $ 3 $ . 3\. Move to floor $ 3 $ : takes $ 2 $ seconds. 4\. Wait for passenger $ 2 $ to arrive: takes $ 4 $ seconds. 5\. Pick up passenger $ 2 $ . 6\. Go to floor $ 2 $ : takes $ 1 $ second. 7\. Pick up passenger $ 1 $ . 8\. Go to floor $ 0 $ : takes $ 2 $ seconds. This gives a total of $ 2+2+4+1+2=11 $ seconds.