S = eval(input())



import string



S = list(sorted(S))

T = string.ascii_lowercase



for t in T:

    if t not in S:

        print(t)

        break

        

else:

    print("None")
S = eval(input())



import string



letters = string.ascii_lowercase



S = set(sorted(S))

for l in letters:

    if l not in S:

        print(l)

        break



else:

    print("None")
S = eval(input())
import string

S = list(sorted(S))
T = string.ascii_lowercase
for t in T:
    if t not in S:
        print(t)
        break
else:
    print("None")

S = eval(input())
import string

letters = string.ascii_lowercase
S = set(sorted(S))
for l in letters:
    if l not in S:
        print(l)
        break
else:
    print("None")

-S = list(sorted(S)) -T = string.ascii_lowercase -for t in T: -    if t not in S: -        print(t) +letters = string.ascii_lowercase +S = set(sorted(S)) +for l in letters: +    if l not in S: +        print(l)