S = eval(input())

wordNum = [0 for i in range(26)]

for wordOne in S:

    idx = ord(wordOne) - ord('a')

    wordNum[idx] += 1



ans = "None"

for i in range(26):

    if wordNum[i] == 0:

        ans = chr(ord('a') + i)

        break;



print(ans)



S = set(eval(input()))

allWord = set('abcdefghijklmnopqrstuvwxyz')



if len(S) == 26:

    print("None")

else:

    print((sorted(allWord - S)[0]))
S = eval(input())
wordNum = [0 for i in range(26)]
for wordOne in S:
    idx = ord(wordOne) - ord("a")
    wordNum[idx] += 1
ans = "None"
for i in range(26):
    if wordNum[i] == 0:
        ans = chr(ord("a") + i)
        break
print(ans)

S = set(eval(input()))
allWord = set("abcdefghijklmnopqrstuvwxyz")
if len(S) == 26:
    print("None")
else:
    print((sorted(allWord - S)[0]))

-S = eval(input()) -wordNum = [0 for i in range(26)] -for wordOne in S: -    idx = ord(wordOne) - ord("a") -    wordNum[idx] += 1 -ans = "None" -for i in range(26): -    if wordNum[i] == 0: -        ans = chr(ord("a") + i) -        break -print(ans) +S = set(eval(input())) +allWord = set("abcdefghijklmnopqrstuvwxyz") +if len(S) == 26: +    print("None") +else: +    print((sorted(allWord - S)[0]))