how to calculate capital gain tax

Hello, I have some doubts about how to calculate capital gain tax on stocks investments when my broker account is outside sweden and I am tax resident in sweden. Let's assume I have some euro in an italian bank and use it to buy a stock using the same bank, and then I sell it realizing a gain in euro. To pay taxes in sweden on the capital gain I have to convert the buy price and the sell price to swedish corona. But which exchange rate do I have to use? Can I use the exchange rate at the moment I sell to convert both the buying and selling price and then calculate the gain? or I have to convert the purchasing price using the exchange rate at the moment I bought and the selling price using the exchange rate at the moment I sold. In this second case when I use 2 different exchange rate, it seems like I am inevitably betting on the swedish korona. Like if I buy for 1.000 euro and sell for 1.000 euro in 1 year, in theory the capital gain is zero. but if in the meanwhile the swedish corona lost half of his value on euro, then I am buying for 10.000 sek and selling for 20.000 sek and I would have to pay some capital gain tax although I didn't realize any gain since I have never converted my money to swedish korona.

In the same situation if instead of buying a stock I keep that 1.000 euro in my italian bank account without buying any stock, in 1 year I will still have 1000 euro but I do not have to pay any tax since I did not buy and sell anything, although I had a gain in sek.