s = list(eval(input()))

for i in 'abcdefghijklmnopqrstuvwxyz':

    if i not in s:

        print(i)

        s = ''

        break

if s != '':

    print('None')

s = eval(input())

for i in 'abcdefghijklmnopqrstuvwxyz':

    if i not in s:

        print(i)

        exit()

print('None')

s = list(eval(input()))
for i in "abcdefghijklmnopqrstuvwxyz":
    if i not in s:
        print(i)
        s = ""
        break
if s != "":
    print("None")

s = eval(input())
for i in "abcdefghijklmnopqrstuvwxyz":
    if i not in s:
        print(i)
        exit()
print("None")

-s = list(eval(input())) +s = eval(input()) -        s = "" -        break -if s != "": -    print("None") +        exit() +print("None")