import string

S = eval(input())

al = string.ascii_lowercase

for x in al:

    if x not in S:

        print(x)

        break

else:

    print("None")
import string

S = eval(input())

eigo = string.ascii_lowercase

eigo_list = list(eigo)



count = 0



for x in eigo_list:

    if not  x in S:

        print(x)

        count += 1

        break

    

if count == 0:

    print("None")

    

import string

S = eval(input())
al = string.ascii_lowercase
for x in al:
    if x not in S:
        print(x)
        break
else:
    print("None")

import string

S = eval(input())
eigo = string.ascii_lowercase
eigo_list = list(eigo)
count = 0
for x in eigo_list:
    if not x in S:
        print(x)
        count += 1
        break
if count == 0:
    print("None")

-al = string.ascii_lowercase -for x in al: -    if x not in S: +eigo = string.ascii_lowercase +eigo_list = list(eigo) +count = 0 +for x in eigo_list: +    if not x in S: +        count += 1 -else: +if count == 0: