You are perhaps going for one of the most difficult streams of M.Sc, which is, of course, Maths. Few of my friends are studying there and as they have mentioned, by the time you reach your third year, one of the subjects in Maths would have a good number of theorems.
The next thing you would have is coordinate geometry as a part of your Maths 1. This would come in handy while solving double/triple integrations.
However, this is still the base of things. It gets complicated ahead- although, not sure if that would be in the third year essentially. Perhaps for better clarity, you should try and reach out to someone who is actually doing his/her M.Sc from BITS Pilani
My friends mentioned that students who are carrying along two degrees at the same time have CS as their ultimate aim. There were a few friends of mine who with a double degree, either ended up slogging into semesters, achieved CS with hardly any effort, and eventually, some of them had to choose chemical/civil out of sheer compulsion.