It's best for undergraduate students to learn mathematics well, which is very helpful for you to study financial engineering, actuarial science and economics in the future! Read math well first, don't think about the second major and postgraduate entrance examination. You can go to graduate school if you study well. Many schools accept students from the Department of Mathematics as graduate students. Mathematics in the department of mathematics is really different from that in the department of non-mathematics. You can watch introductory courses such as mathematical analysis and advanced algebra.