Caesar Cipher is early known substitution cipher

Mathematically give each letter a number

a b c d e f g h i j k l m n o p q r s t u v w x y z

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

then we have Caesar cipher as:

c = E(p) = (p + k) mod (26) and p = D(c) = (c k) mod (26)

(a) With k=17, please decrypt the following ciphertext KYFIE SZIUJ

(b) With k =23, encryption Spring