cf 325 A

A. Alena’s Schedule
time limit per test1 second
memory limit per test256 megabytes
inputstandard input
outputstandard output
Alena has successfully passed the entrance exams to the university and is now looking forward to start studying.

One two-hour lesson at the Russian university is traditionally called a pair, it lasts for two academic hours (an academic hour is equal to 45 minutes).

The University works in such a way that every day it holds exactly n lessons. Depending on the schedule of a particular group of students, on a given day, some pairs may actually contain classes, but some may be empty (such pairs are called breaks).

The official website of the university has already published the schedule for tomorrow for Alena’s group. Thus, for each of the n pairs she knows if there will be a class at that time or not.

Alena’s House is far from the university, so if there are breaks, she doesn’t always go home. Alena has time to go home only if the break consists of at least two free pairs in a row, otherwise she waits for the next pair at the university.

Of course, Alena does not want to be sleepy during pairs, so she will sleep as long as possible, and will only come to the first pair that is presented in her schedule. Similarly, if there are no more pairs, then Alena immediately goes home.

Alena appreciates the time spent at home, so she always goes home when it is possible, and returns to the university only at the beginning of the next pair. Help Alena determine for how many pairs she will stay at the university. Note that during some pairs Alena may be at the university waiting for the upcoming pair.

Input
The first line of the input contains a positive integer n (1 ≤ n ≤ 100) — the number of lessons at the university.

The second line contains n numbers ai (0 ≤ ai ≤ 1). Number ai equals 0, if Alena doesn’t have the i-th pairs, otherwise it is equal to 1. Numbers a1, a2, …, an are separated by spaces.

Output
Print a single number — the number of pairs during which Alena stays at the university.

Sample test(s)
input
5
0 1 0 1 1
output
4
input
7
1 0 1 0 0 1 0
output
4
input
1
0
output
0
Note
In the first sample Alena stays at the university from the second to the fifth pair, inclusive, during the third pair she will be it the university waiting for the next pair.

In the last sample Alena doesn’t have a single pair, so she spends all the time at home.

本题总结来说就是输入一个n表示接下来会有几个输入，然后接下来输入的n个数非0即1。有几个1代表有几节课要上。如果在两个1中间只有1个0.那么这个0会变成1加到最终结果中。如果中间的0为两个或以上，那么这几个0不会变成1。所以代码如上。

我再第一开始考虑问题的时候将算法复杂化了。想了判断如果开始的时候都是0就要都舍去，两个1中间如果是1个0就sum+=1，如果是多个就不操作。这样的代码不易写出、不易读懂。