import string

a = set(string.ascii_lowercase) - set(eval(input()))

if a == set():

    print("None")

else:

    print((min(a)))
a = set("abcdefghijklmnopqrstuvwxyz") - set(eval(input()))

if a == set():

    print("None")

else:

    print((min(a)))
import string

a = set(string.ascii_lowercase) - set(eval(input()))
if a == set():
    print("None")
else:
    print((min(a)))

a = set("abcdefghijklmnopqrstuvwxyz") - set(eval(input()))
if a == set():
    print("None")
else:
    print((min(a)))

-import string - -a = set(string.ascii_lowercase) - set(eval(input())) +a = set("abcdefghijklmnopqrstuvwxyz") - set(eval(input()))