def sum_series(n, n1=0, n2=1):
    """Find nth value in a mathematical series, starting with 2 arbitrary numbers.

    input: n (int) n for nth value in sequence
    input: n1 (int) optional, represents val at sequence n=0
    input: n2 (int) optional, represents val at sequence n=1
    returns: (int) representing value at sequence n
    """
    if n < 1:
        return n1
    elif n == 1:
        return n2
    return sum_series(n - 1, n1, n2) + sum_series(n - 2, n1, n2)