We can apply some common sense to verify the computer's answer. Keep in mind, for a fair dice, each number is equally likely to show after a roll. Thus, each number has a 1 in 6 chance of showing, or a probability of $1/6$.
The chance of rolling a 4, 5 or 6, is $3\times 1/6$ or $3/6$ or $1/2$ or $0.5$. So,
d1 in the above code should result in around this number. For large values of
N, like $1,000$ or so, $0.5$ should result.
The chance of rolling a 1, 2, 3 or 4 us $4\times 1/6$ or $4/6$ or $2/3$ or 0.66. So,
d2 in the above code should result in around this number. For large values of
N, like 1,000 or so, $0.66$ should result. Note the repeated look for a "4" between the two rolls has nothing to do with the fact that the dice is "fair" and all numbers have a 1 in 6 chance of coming up.
Lastly, we address the "and" by multiplying
d1 and
d2 for the final answer to our question.