As a complement to this advice (which I think is good), it's important to make sure you still explore. Don't be so worried about making sure you do the thing that maximizes optionality that you're afraid to fail and don't try things.

So if you think you should study math rather than econ (as per Kaj's comment), then start with math as your default, but make sure to also take an econ class to see if you're so much more interested in it / better at it that it's worth it to specialize.