我的“凯撒密码”

str=input()
a='abcdefghijklmnopqrstuvwxyz'
b='ABCDEFGHIJKLMNOPQRSTUVWXYZ'
for i in range(len(str)):
    if str[i] == " ":
        print(" ", end="")
    elif str[i] in a:
        for j in range(len(a)):
                if str[i]==a[j]:
                    print(a[(j+3)%26],end="")
    elif str[i] in b:
        for k in range(len(b)):
                if str[i]==b[k]:
                    print(b[(k+3)%26],end="")
    else:
        print(str[i],end="")