s=eval(input())

for i in range(26):

  if s.count(chr(ord('a')+i))==0:

    print((chr(ord('a')+i)))

    break

else:

  print('None')
s=eval(input())

for i in range(26):

  if not chr(ord('a')+i) in s:

    print((chr(ord('a')+i)))

    break

else:

  print('None')

s = eval(input())
for i in range(26):
    if s.count(chr(ord("a") + i)) == 0:
        print((chr(ord("a") + i)))
        break
else:
    print("None")

s = eval(input())
for i in range(26):
    if not chr(ord("a") + i) in s:
        print((chr(ord("a") + i)))
        break
else:
    print("None")

-    if s.count(chr(ord("a") + i)) == 0: +    if not chr(ord("a") + i) in s: