Xenia and Ringroad

# 题意 Xenia住在一个城市里，那里所有的房子都是建在一个环线上的，Xenia现在在1号房子里，她有m个任务要完成，并只能按顺时针方向走（房子排列号码也为顺时针递增）。若她要完成第i个任务，她将要走到第ai个房里。请问她要最少要走多少个单位长度？（若两个任务在一个房子里，则可以不走）。 ### 输入 第一行2个数：第一个为城市里房子的总个数，第二个为有几个任务。 ### 输出 她最少要走的步数。 ### 范围 （2<=房子个数<=100000) (1<=任务数<=100000）

Xenia lives in a city that has $ n $ houses built along the main ringroad. The ringroad houses are numbered 1 through $ n $ in the clockwise order. The ringroad traffic is one way and also is clockwise. Xenia has recently moved into the ringroad house number 1. As a result, she's got $ m $ things to do. In order to complete the $ i $ -th task, she needs to be in the house number $ a_{i} $ and complete all tasks with numbers less than $ i $ . Initially, Xenia is in the house number 1, find the minimum time she needs to complete all her tasks if moving from a house to a neighboring one along the ringroad takes one unit of time.

The first line contains two integers $ n $ and $ m $ $ (2<=n<=10^{5},1<=m<=10^{5}) $ . The second line contains $ m $ integers $ a_{1},a_{2},...,a_{m} $ $ (1<=a_{i}<=n) $ . Note that Xenia can have multiple consecutive tasks in one house.

Print a single integer — the time Xenia needs to complete all tasks. Please, do not use the %lld specifier to read or write 64-bit integers in С++. It is preferred to use the cin, cout streams or the %I64d specifier.

4 3
3 2 3

6

4 3
2 3 3

2

In the first test example the sequence of Xenia's moves along the ringroad looks as follows: $ 1→2→3→4→1→2→3 $ . This is optimal sequence. So, she needs 6 time units.