Dear Alois,
We start with a debt of 250
At the end of the year, we owe the same debt plus interest (250 + 0.08*250 = 1.08*250)
At the beginning of the second year, we owe 1.08*250
At the end of the second year, we need to pay interest on that debt: hence, the total debt payment is 250*1.08^2
If 250 is the principal, the remaining amount has to be interest.