This has been the best game I have played so far. The main mechanic is tied into both the countdown and the platforming, which I thought was very interesting.
If I were to make any changes, it'd be to increase the weight of the blocks. Currently they are so light that they can be flung across the entire level on accident. There were a few times where I tried to build a bridge, one block gets nudged slightly and then everything falls down.
Now that I think about it, this game kind of reminds me of Poly Bridge.
There is also a noticeable difficulty spike between levels (especially the last level).
Perhaps for a future update you could add some kind of level editor. It'd be fun to play around with choosing what obstacles to avoid and what numbers we get (possibly also some other operators than just +).
Overall, this is a great game, especially for your first jam! Keep making games!