In a production process, a product is assembled by using four independent parts (A, B, C, and D). In order for the part to operate properly, each part must be free of defects. The probability that the parts are defect-free is p(A) = 0.9, p(B) = 0.7, p(C) = 0.8, and p(D) = 0.9. What is the probability that all four parts have defects?