iredei,
It is incredibly unlikely that TurboTax cannot add and subtract, so let's back up. You list 3 numbers but do not show where you got them from. While I assume you are listing Proceeds, Cost Basis, Gain/Loss, they are not so labelled in your post. First of all, did you get a 1099-B and entered all the numbers about the Bank of Montreal info by hand? Or did you try to import them electronically? If the latter, please obtain the 1099-B printout and, in both cases, provide the actual numbers that appeared on it along with their description. There could be additional information such as wash sales or other adjustments that went into determining the amount of gain or loss for tax purposes. (By the way, Box B is pretty rare as US-based brokers were required to keep track of basis since 2010 or 2011.)