An American roulette wheel has 18 black numbers, 18 red numbers, and two green zeroes ("0" and "00"). A European roulette wheel has only one green zero. There are many possible bets you can make in roulette, but we will focus on players who bet on either ' 'red' or 'black.' If you bet, say, $100 on 'red', then you get your money back plus another $100 if any red number comes up, but you lose your $100 if a green or black number comes up.

i thought it's Ex=18/38*200+20/38*(-100), why is the -100 not correct?