S = eval(input())

list = []

for i in range(len(S)):

    list.append(S[i])

list = set(list)

lists = set([chr(i) for i in range(ord('a'), ord('z')+1)])

ans = lists-list

print(('None' if len(ans)==0 else min(ans)))
S = set(eval(input()))

lists = set([chr(i) for i in range(ord('a'), ord('z')+1)])

ans = lists-S

print((min(ans) if len(list(ans)) != 0 else 'None'))
S = eval(input())
list = []
for i in range(len(S)):
    list.append(S[i])
list = set(list)
lists = set([chr(i) for i in range(ord("a"), ord("z") + 1)])
ans = lists - list
print(("None" if len(ans) == 0 else min(ans)))

S = set(eval(input()))
lists = set([chr(i) for i in range(ord("a"), ord("z") + 1)])
ans = lists - S
print((min(ans) if len(list(ans)) != 0 else "None"))

-S = eval(input()) -list = [] -for i in range(len(S)): -    list.append(S[i]) -list = set(list) +S = set(eval(input())) -ans = lists - list -print(("None" if len(ans) == 0 else min(ans))) +ans = lists - S +print((min(ans) if len(list(ans)) != 0 else "None"))