My advice would be to get just a minor in Math so you can get some easy electives in. if you're considering a career change out of computer science as a possibility and just creating options then there are very few grad schools that would accept a math major but not a math minor. In the job market, a CS plus math minor is probably not going to be a big difference from a double major. This may not be the case for all possibilities but it should hold for most of them. Certainly most employers are going to compliment you then shrug for programming jobs.