 It's a really big problem, because as far as I can tell, they are coming from bases that are very difficult for the Ukrainians to range, or they don't have the authority to use the weapons they do have to attack Russian airfields, with the exception of a few in Crimea. So this is an ongoing challenge, and it will become increasingly expensive and prohibitive to try to strike Russian aircraft while they're airborne to do this. Dara Masako is from the Carnegie Endowment for International Peace. She's a senior fellow.