abc = set('abcdefghijklmnopqrstuvwxyz')

S = set(eval(input()))



print((sorted(abc - S)[0] if len(abc - S) != 0 else None))
alfa = 'abcdefghijklmnopqrstuvwxyz'

S = eval(input())

for i in range(len(alfa)):

    if alfa[i] not in S:

        print((alfa[i]))

        break

else:

    print('None')
abc = set("abcdefghijklmnopqrstuvwxyz")
S = set(eval(input()))
print((sorted(abc - S)[0] if len(abc - S) != 0 else None))

alfa = "abcdefghijklmnopqrstuvwxyz"
S = eval(input())
for i in range(len(alfa)):
    if alfa[i] not in S:
        print((alfa[i]))
        break
else:
    print("None")

-abc = set("abcdefghijklmnopqrstuvwxyz") -S = set(eval(input())) -print((sorted(abc - S)[0] if len(abc - S) != 0 else None)) +alfa = "abcdefghijklmnopqrstuvwxyz" +S = eval(input()) +for i in range(len(alfa)): +    if alfa[i] not in S: +        print((alfa[i])) +        break +else: +    print("None")