Lists of lists in a function


I am running my code, below.

I get the following error on the console window,
Traceback (most recent call last):
File "python", line 10, in
File "python", line 7, in flatten
TypeError: can only concatenate list (not "int") to list

n = [[1, 2, 3], [4, 5, 6, 7, 8, 9]]
# Add your function here
def flatten(lists):
    results = []
    for numbers in lists:
        for i in numbers:
            results = results + i
    return results

print flatten(n)

What is wrong with my code? 
Please help.



results = results + i

would have worked it results was a string or integer, but it is a list, and to add items to a list you need the append function: