Bought my first Casio watch, got a bit bamboozled

I bought my first Casio today on holiday in Germany. I showed the lady behind the counter a picture of a B640, because I read that it has a countdown timer, which I really wanted for my gym sessions. She said that they didn't have it in black, so she brought out a dark metal one, which apparently was an A158. Me being an absolute Casio noob did not notice this and bought it regardless. (I asked her about the countdown but I think In accidentally said 'stopwatch', thinking it could do both).

I do like the watch tho, only thing is I will have to keep an eye on my watch during rest in the gym.