def fibonacci(n):
    """Fibonacci series

    0, 1, 1, 2, 3, 5, 8, 13, ...
    """
    if n <= 1:
        return n
    a = 0
    b = 1
    c = 1
    for i in range(2, n):
        a = b
        b = c
        c = a + b
    return c