s=eval(input())

al=[chr(ord('a') + i) for i in range(26)]

sl=set([s[i] for i in range(len(s))])

for si in sl:

    al.remove(si)

if len(al)>0:

    print((al[0]))

else:

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

al='abcdefghijklmnopqrstuvwxyz'

for a in al:

    if not a in s:

        print(a)

        exit()

print('None')
s = eval(input())
al = [chr(ord("a") + i) for i in range(26)]
sl = set([s[i] for i in range(len(s))])
for si in sl:
    al.remove(si)
if len(al) > 0:
    print((al[0]))
else:
    print("None")

s = eval(input())
al = "abcdefghijklmnopqrstuvwxyz"
for a in al:
    if not a in s:
        print(a)
        exit()
print("None")

-al = [chr(ord("a") + i) for i in range(26)] -sl = set([s[i] for i in range(len(s))]) -for si in sl: -    al.remove(si) -if len(al) > 0: -    print((al[0])) -else: -    print("None") +al = "abcdefghijklmnopqrstuvwxyz" +for a in al: +    if not a in s: +        print(a) +        exit() +print("None")