What do you think the values of int and int are where you are calling it in int main? When you use a function that expects parameters, you should pass it variables that have the same corresponding type. Take a look at the addition function here: http://www.cplusplus.com/doc/tutorial/functions/
With your function body as it is, you could just declare int a and int b as local variables instead of as a paramater.
Your function is also missing a return type. Even if you plan to not return anything, you must put void.
I think you should take a gander at the tutorial I linked to above, so that you can understand how functions work in C++ a bit better.