求斐波那契数列截至第n项时，所有的偶数和

'''
求斐波那契数列截至第n项时，所有的偶数和
'''

def solution(n):

    if n == 0 or n == 1:
        return 0

    total = 0
    i, j = 0, 1 # a[0] = 0, a[1] = 1
    m = 2 # 从第二项开始
    while m <= n:
        s = i +j
        if s % 2 == 0:
            total += s
        # 更新
        i = j
        j = s
        m += 1

    return total


if __name__ == '__main__':
    print(solution(10))