As per Luke's request and Peter Taylor's addition to this challenge.

###Introduction

Everyone knows the game tic-tac-toe, but in this challenge, we are going to introduce a little twist. We are only going to use crosses. The first person who places three crosses in a row loses. An interesting fact is that the maximum amount of crosses before someone loses, is equal to 6:

X X -
X - X
- X X

That means that for a 3 x 3 board, the maximum amount is 6. So for N = 3, we need to output 6.

Another example, for N = 4, or a 4 x 4 board:

X X - X
X X - X
- - - -
X X - X

This is an optimal solution, you can see that the maximum amount of crosses is equal to 9. An optimal solution for a 12 x 12 board is:

X - X - X - X X - X X -
X X - X X - - - X X - X
- X - X - X X - - - X X
X - - - X X - X X - X -
- X X - - - X - - - - X
X X - X X - X - X X - -
- - X X - X - X X - X X
X - - - - X - - - X X -
- X - X X - X X - - - X
X X - - - X X - X - X -
X - X X - - - X X - X X
- X X - X X - X - X - X

This results in 74.

###The task

Your task is to compute the results as fast as possible. I will run your code on the test case 13. This will be done 5 times and then the average is taken of the run times. That is your final score. The lower, the better.

###Test cases

N     Output
1       1
2       4
3       6
4       9
5       16
6       20
7       26
8       36
9       42
10      52
11      64
12      74
13      86
14      100
15      114

More information can be found at https://oeis.org/A181018.

###Rules

• This is fastest-code, so the fastest submission wins!
• You must provide a full program.
• Please also provide how I have to run the program. I am not familiar with all programming languages and how they work, so I need a little bit of help here.
• Of course, your code needs to compute the correct result for each test case.

###Submissions:

• feersum (C++11): 28s
• Peter Taylor (Java): 14m 31s