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.

Example Input: [217.301, 19.358, -21.02, -51.249]
Example Output: 41.098

Example Input: [129.453, 232.284, -34.595]
Example Output: 109.047

Example Input: [155.723, 143.973, -89.725, -63.838, 82.645, -82.46, 119.929, 42.78, 230.62]
Example Output:
59.961