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"""

Project Euler Problem 2: https://projecteuler.net/problem=2

Even Fibonacci Numbers

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

four million, find the sum of the even-valued terms.

References:

- https://en.wikipedia.org/wiki/Fibonacci_number

"""

def solution(n: int = 4000000) -> int:

"""

Returns the sum of all even fibonacci sequence elements that are lower

or equal to n.

>>> solution(10)

>>> solution(15)

>>> solution(2)

>>> solution(1)

>>> solution(34)

"""

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(f"{solution() = }")

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