 So this one asks us to calculate the degree of unsaturation in the following formula. And then it tells us that this formula belongs to caffeine C8H10N4O2. So if you recall, and if you don't, it's OK. Because I'm going to have to teach you. Whenever you have the halogen, which we don't have in this one, we'll say x. You add one hydrogen to it. Add one hydrogen to the hydrogen. If you have oxygen down the calcogens, so oxygen and sulfur are the two main ones. Like that, you don't do anything. You don't add any hydrogen. You add zero hydrogen. Then nitrogen and phosphorus on down. You subtract one hydrogen from that. And you recall the formula for unsaturated alkane. What is that? So C and H2 and plus 2. Like that. So what we're going to do here is use these rules, take these atoms off, add and subtract those atoms, and then use that C and H2 and plus 2. This molecular formula says that we have two oxygens. What are we supposed to do to the number of hydrogens? Don't do anything, right? So we're going to just cancel those out. What about nitrogens? What are we supposed to do for each nitrogen? Subtract one hydrogen. So since we've got four of them, we're going to subtract four hydrogens. So our new formula is C8H6. Like that. That makes sense, right? Because we subtracted the four hydrogens. So CnH2n plus 2 tells us that if we have C8, we should have how many Hs? H2n, that's 16, right? Plus 2 is 18. So that should be our formula for the saturated hydrocarp. But in fact, we have C8H6. So obviously every time that you do this, the targets are going to cancel out. So we have 18 minus 6. What is that? 12. Very good. And remember, we're doing this for every two hydrogens. So every two hydrogens is one unit of unsaturation. So if we've got 12, how many units of unsaturation are degrees? Six degrees of unsaturation. Any questions on that one? Wonderful.