Consider a Feistel cipher with four rounds. Then the plaintext is denoted as P $=($ L$0,$ R$0)$ and the corresponding ciphertext is C $=($ L$4,$ R$4).$ What is the simplest form of the ciphertext C$,$ in terms of L$0,$ R$0,$ and the subkeys, for the round function F$($ Ri$-1,$ Ki $)=$ Ki $?$