Definition: In this task you will be given a list of numbers and you need to find the mean (average) of that list. The mean of a list can be found by summing every number in the list then dividing the result by the size of that list. The output should be rounded to 3 decimal places.
Input: [-66.387, 135.639, 182.921, 175.573, 105.67, -29.751, 156.67, 83.119, 19.538]

84.777