def ForwardDiff(df, n=1):
    """Calculate the difference of value after n rows.

    Parameters
    ----------
    df : pandas DataFrame
    n : int

    Returns
    -------
    ret : DataFrame.
    
    """
    ret = df.diff(periods=n)
    ret = ret.shift(periods= -1 * n)
    ret = ret.dropna()
    return ret