this code should calculate the summation of all the even number in a Fibonacci sequence that will be generated automaticlly, while the max number of the generated sequence do not exceed four million if not givin the max number by the user as an argument.
by the way Fibonacci sequence is generated by adding the previous two terms. By starting with 1 and 2. 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...