Change score in long format

Hey guys.

So I'm working on data and need to compute a change score. The data is panel data, where ID is country and I have 12 years. Here's my problem. I need to drop all years between 2000 and 2011 and compute a change score between 2000 and 2011. The drop part isn't the problem but the change score is. Because I have year in columns, I can't just subtract year 2000 with 2011 because the variable is listed in one. For example; I have count of apples in Romania for 2000 and 2011 and I want to have a new variable that contains the two years quantity subtracted to use in a regression model but I don't know how. How can I subtract apples in year 2000 from apples in year 2011 if the data is in long format? Perhaps I need to use the lead-function?