def subtract_float_columns_simple(df, A, B):
    # Convert columns to float, set invalid parsing to NaN
    df[A] = pd.to_numeric(df[A], errors='coerce')
    df[B] = pd.to_numeric(df[B], errors='coerce')

    # Subtract the columns, NaN will be propagated automatically
    return df[B] - df[A]