Unfortunately there was a mistake in the ILP of last week's lecture; rather than Behind(i,j), the sum should be over CCW(i,j), where CCW(i,j) are the vertices to the "right" of the line through i and j. (That is, points k such that the angle i-j-k is positive between 0 and 180.) This change is actually required for the correctness of the ILP.
(If you have already handed in your solutions with a "proof" for the current exercise 2, we will grade it "in spirit" and give points based on your fair assumption that we were asking you to prove something true.)
I am away this week and will update the slides next week.