CF1000B Light It Up

**题目大意：** 有一台灯，这个灯在时间为$0$时打开，$m$时关闭，在$0$到$m$这段时间内有$n$个时间点灯的状态会改变（即开变关，关变开），现在可以在**剩余**的时间点选一个让灯的状态改变一次，求这个灯最大亮着的时间

第一行两个整数，$n,m$，$n

一个整数，即最大亮着的时间 感谢@守望 提供翻译

In the first example, one of possible optimal solutions is to insert value $ x = 3 $ before $ a_1 $ , so program will be $ [3, 4, 6, 7] $ and time of lamp being lit equals $ (3 - 0) + (6 - 4) + (10 - 7) = 8 $ . Other possible solution is to insert $ x = 5 $ in appropriate place. In the second example, there is only one optimal solution: to insert $ x = 2 $ between $ a_1 $ and $ a_2 $ . Program will become $ [1, 2, 10] $ , and answer will be $ (1 - 0) + (10 - 2) = 9 $ . In the third example, optimal answer is to leave program untouched, so answer will be $ (3 - 0) + (7 - 4) = 6 $ .