S = eval(input())

ans='None'

for i in range(26):

  s = chr(ord('a') + i)

  if s not in S:

    ans = s

    break

print(ans)



#print(*ans, sep='\n')

def solve():

    ans = 0

    S = eval(input())

    lis = [0]*26

    for s in S:

        lis[ord(s)-ord('a')] += 1

    for i,l in enumerate(lis):

        if l==0:

            ans = chr(ord('a')+i)

            return ans

    return 'None'

print((solve()))
S = eval(input())
ans = "None"
for i in range(26):
    s = chr(ord("a") + i)
    if s not in S:
        ans = s
        break
print(ans)
# print(*ans, sep='\n')

def solve():
    ans = 0
    S = eval(input())
    lis = [0] * 26
    for s in S:
        lis[ord(s) - ord("a")] += 1
    for i, l in enumerate(lis):
        if l == 0:
            ans = chr(ord("a") + i)
            return ans
    return "None"


print((solve()))

-S = eval(input()) -ans = "None" -for i in range(26): -    s = chr(ord("a") + i) -    if s not in S: -        ans = s -        break -print(ans) -# print(*ans, sep='\n') +def solve(): +    ans = 0 +    S = eval(input()) +    lis = [0] * 26 +    for s in S: +        lis[ord(s) - ord("a")] += 1 +    for i, l in enumerate(lis): +        if l == 0: +            ans = chr(ord("a") + i) +            return ans +    return "None" + + +print((solve()))