I need a mathematician.

If I have a random sequence of bits being received from a datastream, how can I determine the probability that after n bits I will have received at least 1 sequence of r consecutive 1s?

By analysis I can determine that after 3 bits are received the probability of getting 3 consecutive 1s is 1/8, after 4 bits it's 3/16 and after 5 bits it's 8/32.

What is the general formula?

Regards

Mintguy