Explain all calculation steps

Consider a frame consisting of four characters of 7 bits each. Assume that the probability of bit error is 10~6 and that it is independent for each bit. What is the probability that the received frame contains at least one error? Now add a parity bit for the frame and determine the probability. Finally instead of a parity bit for the frame, add a parity bit to each character. What is then the probability of a frame error