S = eval(input())

alphabet = "abcdefghijklmnopqrstuvwxyz"

found = [i for i in alphabet if i not in S]

found.sort()

if len(found) == 0:

  print('None')

else:

  print((found[0]))
S = eval(input())

alphabet = "abcdefghijklmnopqrstuvwxyz"

found = False

for i in alphabet:

  if i not in S:

    print(i)

    found = True

    break



if not found:

    print('None')
S = eval(input())
alphabet = "abcdefghijklmnopqrstuvwxyz"
found = [i for i in alphabet if i not in S]
found.sort()
if len(found) == 0:
    print("None")
else:
    print((found[0]))

S = eval(input())
alphabet = "abcdefghijklmnopqrstuvwxyz"
found = False
for i in alphabet:
    if i not in S:
        print(i)
        found = True
        break
if not found:
    print("None")

-found = [i for i in alphabet if i not in S] -found.sort() -if len(found) == 0: +found = False +for i in alphabet: +    if i not in S: +        print(i) +        found = True +        break +if not found: -else: -    print((found[0]))