a=list('abcdefghijklmnopqrstuvwxyz')

s=list(eval(input()))

for i in s:

    if a.count(i):

        a.remove(i)

if len(a)==0:

    print('None')

else:

    print((a[0]))
# 入力

S=eval(input())



# a〜z

l=['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']



for x in l:

  if x not in S:

    print(x)

    exit()



print('None')
a = list("abcdefghijklmnopqrstuvwxyz")
s = list(eval(input()))
for i in s:
    if a.count(i):
        a.remove(i)
if len(a) == 0:
    print("None")
else:
    print((a[0]))

# 入力
S = eval(input())
# a〜z
l = [
    "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",
]
for x in l:
    if x not in S:
        print(x)
        exit()
print("None")

-a = list("abcdefghijklmnopqrstuvwxyz") -s = list(eval(input())) -for i in s: -    if a.count(i): -        a.remove(i) -if len(a) == 0: -    print("None") -else: -    print((a[0])) +# 入力 +S = eval(input()) +# a〜z +l = [ +    "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", +] +for x in l: +    if x not in S: +        print(x) +        exit() +print("None")