I'll agree to those terms. I'd also take a 1/5th chance of $100 in lieu of $20 (e.g. you pay $100 if no Russian tanks have been involved in combat in the Ukraine by April 28th and the first digit after the decimal of the S&P 500 is a 0 or 5 at close that day).
I think it's a tie since tanks were not involved in combat (although they did invade) but the S&P 500 Index closed at 1,869.43 on April 28th.
A Dan Carlin Podcast about how the United States is foolishly antagonizing the Russians over Ukraine. Carlin makes an analogy as to how the United States would feel if Russia helped overthrow the government of Mexico to install an anti-American government under conditions that might result in a Mexican civil war. Because of the Russian nuclear arsenal, even a tiny chance of a war between the United States and Russia has a huge negative expected value.