"""
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.
"""

UP_NUM = 4000000


def is_even(num: int) -> bool:
    """Check if a number is even."""
    return num & 1 == 0


"""
下面这个方法实在太慢，计算400万项还会内存溢出。
```python
def fibonacci_sequence(n: int) -> list[int]:
    a, b = 1, 2
    res = [a, b]
    while b <= n:
        a, b = b, a + b
        res.append(b)
    return res


def main():
    fib_seq = fibonacci_sequence(UP_NUM)
    even_sum = sum(num for num in fib_seq if is_even(num))
    print(even_sum)
```
"""


def even_fibonacci(limit: int) -> int:
    a, b = 1, 2
    even_sum = 0
    while b <= limit:
        if is_even(b):
            even_sum += b
        a, b = b, a + b
    return even_sum


if __name__ == "__main__":
    print(even_fibonacci(UP_NUM))