s = list(eval(input()))

c = 0

for i in range(97,97 + 26):

	if chr(i) not in  s:

		print((chr(i)))

		break

	c += 1

if c == 26:

  print('None')
s = eval(input())

for i in range(97,97 + 26):

	if chr(i) not in  s:

		print((chr(i)))

		exit()

print('None')
s = list(eval(input()))
c = 0
for i in range(97, 97 + 26):
    if chr(i) not in s:
        print((chr(i)))
        break
    c += 1
if c == 26:
    print("None")

s = eval(input())
for i in range(97, 97 + 26):
    if chr(i) not in s:
        print((chr(i)))
        exit()
print("None")

-s = list(eval(input())) -c = 0 +s = eval(input()) -        break -    c += 1 -if c == 26: -    print("None") +        exit() +print("None")