CF 339B Xenia and Ringroad

题目地址：http://codeforces.com/problemset/problem/339/B
B. Xenia and Ringroad
time limit per test2 seconds
memory limit per test256 megabytes
inputstandard input
outputstandard output
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 ai 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.

Input
The first line contains two integers n and m (2 ≤ n ≤ 105, 1 ≤ m ≤ 105). The second line contains m integers a1, a2, …, am (1 ≤ ai ≤ n). Note that Xenia can have multiple consecutive tasks in one house.

Output
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.

Examples
inputCopy
4 3
3 2 3
outputCopy
6
inputCopy
4 3
2 3 3
outputCopy
2
Note
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.

解题思路：一共有n座房子，房子形成一个循环的圈，有一条单向的路供人行走。A君从1号房子开始，想起自己要做M件事情，接下去输入每件事情要在几号房子完成，求出A君做完这些事情需要的最少路程。
这题直接用循环的思想求路径长度，这题wa了一次，应为数据都取得Int，没有考虑到，就算n和m没有超出Int,但是总路径ans可能会爆了。

#include 
using namespace std;
int main()
{
	long long int n,m;
	while(cin >> n >> m)
	{
		long long int ans = 0;
		long long int first = 1;
		while(m--)
		{
			long long num;
			cin >> num;
			ans += (num+n-first)%n;
			first = num;
		}
		cout<