C++ :: Unexpected Output In Example Function

I am currently trying to understand why this example for using an array as an argument in a function has a different result than what the lecture notes say it should be.

So supposedly sum should return with the value 28, but I get 27 no matter what. I also am not very good at reading and understanding what exactly the order of operations for this function are.

Code:
#include <iostream>
using namespace std;

int sum(const int array[], const int length) {
long sum = 0;
for (int i = 0; i < length; sum += array[++i]);
return sum;

[Code] ....