Python/project_euler/problem_002/sol3.py

"""
Problem:
Each new term in the Fibonacci sequence is generated by adding the previous
two terms. By starting with 1 and 2, the first 10 terms will be:

    1,2,3,5,8,13,21,34,55,89,..

By considering the terms in the Fibonacci sequence whose values do not exceed
n, find the sum of the even-valued terms. e.g. for n=10, we have {2,8}, sum is
10.
"""


def solution(n: int = 4000000) -> int:
    """Returns the sum of all fibonacci sequence even elements that are lower
    or equals to n.

    >>> solution(10)
    10
    >>> solution(15)
    10
    >>> solution(2)
    2
    >>> solution(1)
    0
    >>> solution(34)
    44
    """
    if n <= 1:
        return 0
    a = 0
    b = 2
    count = 0
    while 4 * b + a <= n:
        a, b = b, 4 * b + a
        count += a
    return count + b


if __name__ == "__main__":
    print(solution(int(input().strip())))