You are given a string S consisting of lowercase English letters. Find the lexicographically (alphabetically) smallest lowercase English letter that does not occur in S. If every lowercase English letter occurs in S, print `None` instead.
S = input()
List = []
for i in range(len(S)):
  tmp = S[i]
  List.append(tmp)
List = list(set(List))
num2alpha = lambda c: chr(c+64)
for k in range(1,27):
  if(num2alpha(k) in List):
    print(num2alpha(k))
    exit()
print("None")
S = input()
List = []
for i in range(len(S)):
  tmp = S[i]
  List.append(tmp)
List = list(set(List))
num2alpha = lambda c: chr(c+96)
for k in range(1,27):
  if(num2alpha(k) in List):
    pass
  else:
    print(num2alpha(k))
    exit()
print("None")