S = eval(input())

alphabets = set(list('abcdefghijklmnopqrstuvwxyz'))



for s in S:

    alphabets.discard(s)



if len(alphabets) != 0:

    print((sorted(alphabets)[0]))

else:

    print('None')
S = set(eval(input()))

diff = set('abcdefghijklmnopqrstuvwxyz') - S

if diff:

    print((min(diff)))

else:

    print('None')
S = eval(input())
alphabets = set(list("abcdefghijklmnopqrstuvwxyz"))
for s in S:
    alphabets.discard(s)
if len(alphabets) != 0:
    print((sorted(alphabets)[0]))
else:
    print("None")

S = set(eval(input()))
diff = set("abcdefghijklmnopqrstuvwxyz") - S
if diff:
    print((min(diff)))
else:
    print("None")

-S = eval(input()) -alphabets = set(list("abcdefghijklmnopqrstuvwxyz")) -for s in S: -    alphabets.discard(s) -if len(alphabets) != 0: -    print((sorted(alphabets)[0])) +S = set(eval(input())) +diff = set("abcdefghijklmnopqrstuvwxyz") - S +if diff: +    print((min(diff)))