S = list(eval(input()))

L = len(S)

Slist = set()

for i in range(L):

    Slist.add(ord(S[i]))

Slist = list(Slist)

Slist.sort()

ans = "None"

j = 0

L = len(Slist)

for i in range(97, 123):

    if Slist[j] == i:

        j += 1

        if j >= L and i+1 < 123:

            ans = chr(i+1)

            break

        continue

    ans = chr(i)

    break

print(ans)

from collections import Counter

S = list(eval(input()))

S = Counter(S)

if len(S) == 26:

    ans = "None"

else:

    A = [chr(i) for i in range(97, 97+26)]

    for i in A:

        if i in S:

            continue

        ans = i

        break

print(ans)

S = list(eval(input()))
L = len(S)
Slist = set()
for i in range(L):
    Slist.add(ord(S[i]))
Slist = list(Slist)
Slist.sort()
ans = "None"
j = 0
L = len(Slist)
for i in range(97, 123):
    if Slist[j] == i:
        j += 1
        if j >= L and i + 1 < 123:
            ans = chr(i + 1)
            break
        continue
    ans = chr(i)
    break
print(ans)

from collections import Counter

S = list(eval(input()))
S = Counter(S)
if len(S) == 26:
    ans = "None"
else:
    A = [chr(i) for i in range(97, 97 + 26)]
    for i in A:
        if i in S:
            continue
        ans = i
        break
print(ans)

+from collections import Counter + -L = len(S) -Slist = set() -for i in range(L): -    Slist.add(ord(S[i])) -Slist = list(Slist) -Slist.sort() -ans = "None" -j = 0 -L = len(Slist) -for i in range(97, 123): -    if Slist[j] == i: -        j += 1 -        if j >= L and i + 1 < 123: -            ans = chr(i + 1) -            break -        continue -    ans = chr(i) -    break +S = Counter(S) +if len(S) == 26: +    ans = "None" +else: +    A = [chr(i) for i in range(97, 97 + 26)] +    for i in A: +        if i in S: +            continue +        ans = i +        break