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: [-18.016, 247.669, 100.835, 180.329, -11.701, 46.576, 47.896, 175.095, 21.187]

87.763