import itertools

s = eval(input())

cnt = ord('a')

for k, _ in itertools.groupby(sorted(s)):

  if not cnt == ord(k):

    break

  cnt += 1

print(([chr(cnt), 'None'][cnt>ord('z')]))
ans = sorted({chr(i) for i in range(97, 123)} - set(eval(input())))

print((ans[0] if ans else 'None'))
import itertools

s = eval(input())
cnt = ord("a")
for k, _ in itertools.groupby(sorted(s)):
    if not cnt == ord(k):
        break
    cnt += 1
print(([chr(cnt), "None"][cnt > ord("z")]))

ans = sorted({chr(i) for i in range(97, 123)} - set(eval(input())))
print((ans[0] if ans else "None"))

-import itertools - -s = eval(input()) -cnt = ord("a") -for k, _ in itertools.groupby(sorted(s)): -    if not cnt == ord(k): -        break -    cnt += 1 -print(([chr(cnt), "None"][cnt > ord("z")])) +ans = sorted({chr(i) for i in range(97, 123)} - set(eval(input()))) +print((ans[0] if ans else "None"))