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

a=set([chr(i+97) for i in range(26)])

a-=s

print((sorted(list(a))[0] if len(a)!=0 else "None"))
s=eval(input())

a=[chr(i) for i in range(ord("a"),ord("z")+1) if chr(i) not in s]

print(("None" if len(a)==0 else a[0]))
s = set(list(eval(input())))
a = set([chr(i + 97) for i in range(26)])
a -= s
print((sorted(list(a))[0] if len(a) != 0 else "None"))

s = eval(input())
a = [chr(i) for i in range(ord("a"), ord("z") + 1) if chr(i) not in s]
print(("None" if len(a) == 0 else a[0]))

-s = set(list(eval(input()))) -a = set([chr(i + 97) for i in range(26)]) -a -= s -print((sorted(list(a))[0] if len(a) != 0 else "None")) +s = eval(input()) +a = [chr(i) for i in range(ord("a"), ord("z") + 1) if chr(i) not in s] +print(("None" if len(a) == 0 else a[0]))