from string import ascii_lowercase





S = eval(input())

ans = None

for c in ascii_lowercase:

    if c not in S:

        ans = c

        break

print(ans)
S = eval(input())

ans = None

for c in 'abcdefghijklmnopqrstuvwxyz':

    if c not in S:

        ans = c

        break

print(ans)
from string import ascii_lowercase

S = eval(input())
ans = None
for c in ascii_lowercase:
    if c not in S:
        ans = c
        break
print(ans)

S = eval(input())
ans = None
for c in "abcdefghijklmnopqrstuvwxyz":
    if c not in S:
        ans = c
        break
print(ans)

-from string import ascii_lowercase - -for c in ascii_lowercase: +for c in "abcdefghijklmnopqrstuvwxyz":