def avg_vec_of_vecs(vecs, dim=None, size=None):
    """
    Calculcate average vector (center vector) for input vectors <vecs>.
    :param vecs: input vectors
    :param dim: dimension of <vecs> if it was already calculated
    :param size: size of <vecs> if it was already calculated
    :return average vector (center vector) for input vectors <vecs>
    """
    size = size or len(vecs)
    dim = dim or len(vecs[0])
    avg_vec = [0.0] * dim
    for vec in vecs:
        for i, x in enumerate(vec):
            avg_vec[i] += x / size

    return avg_vec