arb = 'abcdefghijklmnopqrstuvwxyz'

S = eval(input())



ans =''

for i in arb:

    if i not in S:

        ans += i



sort_ans = sorted(ans)

if len(ans)==0:

    print('None')

else:

    print((sort_ans[0]))
arb = 'abcdefghijklmnopqrstuvwxyz'

S = eval(input())

s_S =sorted(set(arb)^set(S))

print(('None' if len(s_S) == 0 else s_S[0]))

arb = "abcdefghijklmnopqrstuvwxyz"
S = eval(input())
ans = ""
for i in arb:
    if i not in S:
        ans += i
sort_ans = sorted(ans)
if len(ans) == 0:
    print("None")
else:
    print((sort_ans[0]))

arb = "abcdefghijklmnopqrstuvwxyz"
S = eval(input())
s_S = sorted(set(arb) ^ set(S))
print(("None" if len(s_S) == 0 else s_S[0]))

-ans = "" -for i in arb: -    if i not in S: -        ans += i -sort_ans = sorted(ans) -if len(ans) == 0: -    print("None") -else: -    print((sort_ans[0])) +s_S = sorted(set(arb) ^ set(S)) +print(("None" if len(s_S) == 0 else s_S[0]))