import string

s = eval(input())

for a in string.ascii_lowercase:

    if a not in s:

        print(a)

        break

    if a =="z":

        print("None")

        break
a = sorted(set('abcdefghijklmnopqrstuvwxyz')-set(eval(input())))

print((a[0] if a else "None"))
import string

s = eval(input())
for a in string.ascii_lowercase:
    if a not in s:
        print(a)
        break
    if a == "z":
        print("None")
        break

a = sorted(set("abcdefghijklmnopqrstuvwxyz") - set(eval(input())))
print((a[0] if a else "None"))

-import string - -s = eval(input()) -for a in string.ascii_lowercase: -    if a not in s: -        print(a) -        break -    if a == "z": -        print("None") -        break +a = sorted(set("abcdefghijklmnopqrstuvwxyz") - set(eval(input()))) +print((a[0] if a else "None"))