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.
A=input()
B=sorted(set(A))

def aaa(a):
  return(ord(a))

C=list(map(aaa,B))

for i in range(26):
  if (i+97 in C)!=1:
    print(i+97)
    break
  if i==25:
    print("None")
    
A=input()
B=sorted(set(A))

def aaa(a):
  return(ord(a))

C=list(map(aaa,B))

for i in range(26):
  if (i+97 in C)!=1:
    print(chr(i+97))
    break
  if i==25:
    print("None")
    