s = sorted(list(set(eval(input()))))

a = "abcdefghijklmnopqrstuvwxyz"



if "".join(s) == a:

    print("None")

    exit()

else:

    for i in s:

        a = a.replace(i, "")



print((a[0]))
a = [chr(ord("a") + i) for i in range(26)]

s = sorted(list(set(eval(input()))))



for i in a:

    if i not in s:

        print(i)

        exit()



print("None")
s = sorted(list(set(eval(input()))))
a = "abcdefghijklmnopqrstuvwxyz"
if "".join(s) == a:
    print("None")
    exit()
else:
    for i in s:
        a = a.replace(i, "")
print((a[0]))

a = [chr(ord("a") + i) for i in range(26)]
s = sorted(list(set(eval(input()))))
for i in a:
    if i not in s:
        print(i)
        exit()
print("None")

+a = [chr(ord("a") + i) for i in range(26)] -a = "abcdefghijklmnopqrstuvwxyz" -if "".join(s) == a: -    print("None") -    exit() -else: -    for i in s: -        a = a.replace(i, "") -print((a[0])) +for i in a: +    if i not in s: +        print(i) +        exit() +print("None")