Syntax issue on creating MySQL scalar-value function

I'm trying to create a function in MySQL that returns the sum of each row's difference between two of its decimal values. This statement works correctly:

SELECT SUM(ending_mileage - beginning_mileage) AS total_mileage FROM trip;

When I attempt to put this into a function to be easily accessed in my project, I get syntax errors on the parenthesis after trip saying "Statement is incomplete, expecting ';'" and on the END saying "END is not valid at this position". Is there some kind of syntax that I am not stating correctly?

CREATE FUNCTION fnMileageTotal()
RETURNS DECIMAL
BEGIN
    RETURN (SELECT SUM(ending_mileage - beginning_mileage) FROM trip);
END;