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: [71.39, 133.73, -86.265, 237.963, 228.337, 220.263, 127.766, 174.117]

138.413