Write a function named average that accepts a list of integers as its parameter and returns the average (arithmetic mean) of all elements in the list as a float. For example, if the list passed contains the values [1, -2, 4, -4, 9, -6, 16, -8, 25, -10], the calculated average should be 2.5. You may assume that the list contains at least one element. Your function should not modify the elements of the list.