Formula to calculate the probability of the next value, considering previous pattern

Hello,

I was hoping somebody could help me with a formula...


I have two columns A (where the only values that will be entered are "apple" and "peach") and B. I need a formula to enter in cells of column B that will return the following values: what is the probability of "apple" and "peach" to be the next value in column A, considering the previous "pattern" from column A? Consider the case attached.


Thank you!