def ncoeffs(degree):
    """
    Calculate the number of coefficients in a bivarite polynomail.

    Parameters:

    * degree : int
        The degree of the polynomial

    Returns:

    * n : int
        The number of coefficients

    Examples:

    >>> ncoeffs(1)
    3
    >>> ncoeffs(2)
    6
    >>> ncoeffs(3)
    10
    >>> ncoeffs(4)
    15

    """
    return sum(range(1, degree + 2))