From my experience, i learnt that I didn't have to figure everything out before university or even the few years after. A great way to learn about yourself, your passion and what do you want to when you get older is by experimenting. I recommend that you explore as much as you can with your passion for economics and math and keep your heart open for other fields as well. You will see that soon after things will start unfolding for you in terms of passion and career. I have also changed 3 jobs in my first 3 years after graduation as I was experimenting things that I want to do as my everyday job.
Your question reminds me of something I learned something a while back and want to pass it on to you. There was a group of little children who were asked what they wanted to be when they grew up. The first few children gave expected answers like "A fireman" or "A doctor" but one little boy gave the best and most profound answer. He said "When I grow up I want to be nice and brave". We may chuckle at his answer and assume he did not understand the question, but I believe his answer was correct and the question was wrong. In the pure clarity of this child's mind he understood that a job is what you "do", and is not who you "are".
This experience caused me some personal reflection. Sure, there are career aspirations and goals that I have, but how often do we as individuals or societies actually consider what type of people we want to become. On the surface making a distinction between what we want to be versus what we want to do when we grow up may sound trite or silly, but I believe that it is distinction that matters a great deal.
Our ultimate success and happiness in life is not actually dependent on what we happen to do to earn a paycheck but is found by developing into who we want to really become. A good place to start is by looking at the people around you that you love and respect the most. It is likely that you esteem them highly because of who they are as a person as opposed to a job they happen to have. What traits or characteristics do they have that you would like to emulate? What personal strengths do you already posses? What character flaws or weaknesses do you posses that you want to change? I have found that when I consider these questions and others like them I am able to develop into who I ultimate want to be. Which at the end my my life will matter far more to me and my posterity than what I happened to do to earn money.
I wish you great success and happiness in life and encourage you and all of us to consider what we want to "be" and not just what we want to "do".
I love math as well. I am Senior Finance Officer in a Big 2 Auditing Company for 14 years. I am happy with my work because I have good teammates and workplace. Our company invest their staff to be digitally skills; I enjoyed those trainings; it enhanced my skills and very helpful with my day-to-day work. I believed if you are dedicated and have passion to your work; nothing is impossible to be successful.
Good luck dear and wishing you all the best in your study.
If you do decide to go in as undecided, you can always transfer the credits from your first year as electives, before switiching majors so the credits do not go to waste. Plus, econ and math may be mandatory for a lot of majors anyway.