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: [86.502, -56.025, -44.049, -34.684, 4.776, -57.238, 20.845, 217.299, 23.387]

17.868