Didn't get the formula

Hey Bulat,    Thanks for reaching out!   In the formula, we're using "." to express multiplication, which might be what is confusing you. Overall, we're just multiplying each outcome by its likelihood of occurring (P(X)*X) and adding them up (P(X_1)*X_1 + P(X_2)*X_2).    Hope this helps! Best,  365 Vik