l=set(i for i in "abcdefghijklmnopqrstuvwxyz")

n=set(j for j in eval(input()))

b=sorted(l-n)

print(("None" if b==[] else b[0]))
s=eval(input())

t=list(chr(i) for i in range(97,97+26))  

bools=True

for i in t:

  if i not in s:

    print(i)

    bools=False

    break

if bools:print("None")
l = set(i for i in "abcdefghijklmnopqrstuvwxyz")
n = set(j for j in eval(input()))
b = sorted(l - n)
print(("None" if b == [] else b[0]))

s = eval(input())
t = list(chr(i) for i in range(97, 97 + 26))
bools = True
for i in t:
    if i not in s:
        print(i)
        bools = False
        break
if bools:
    print("None")

-l = set(i for i in "abcdefghijklmnopqrstuvwxyz") -n = set(j for j in eval(input())) -b = sorted(l - n) -print(("None" if b == [] else b[0])) +s = eval(input()) +t = list(chr(i) for i in range(97, 97 + 26)) +bools = True +for i in t: +    if i not in s: +        print(i) +        bools = False +        break +if bools: +    print("None")