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.

-----Constraints-----
 - 1 \leq |S| \leq 10^5 (|S| is the length of string S.)
 - S consists of lowercase English letters.

-----Input-----
Input is given from Standard Input in the following format:
S

-----Output-----
Print the lexicographically smallest lowercase English letter that does not occur in S.
If every lowercase English letter occurs in S, print None instead.

-----Sample Input-----
atcoderregularcontest

-----Sample Output-----
b

The string atcoderregularcontest contains a, but does not contain b.

Difficulty: introductory

Starter code:

Solutions:
S = input()
alphabet = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z']

for a in alphabet:
    if a not in S:
        print(a)
        return
        
print('None')




s = input()
ans = 'None'
for i in range(97, 123):
    if chr(i) not in s:
        ans = chr(i)
        break
print(ans)

S = input()
for c in "abcdefghijklmnopqrstuvwxyz":
    if c not in S:
        print(c)
        break
else:
    print("None")


import string

S = set(input())

result = ''
for i in string.ascii_lowercase:
    if i not in S:
        print(i)
        break
else:
    print('None')

import sys
import copy
import math
import itertools
import numpy as np
S = input()
for i in "abcdefghijklmnopqrstuvwxyz":
    if S.count(i) == 0:
        print(i)
        return

print("None")

S = input()
l = sorted(list(set(S)))

ans = ""
for i in range(len(l)):
  if ord(l[i]) !=  97 + i:
    ans = chr(97+i)
    break
  elif i == 25 and l[i] == "z":
    ans = "None"

if ans != "":
 print(ans)
else:
  print(chr(ord(l[-1])+1))

S = str(input())
s = 'abcdefghijklmnopqrstuvwxyz'

for i in s:
  if i not in S:
    print(i)
    return
print('None')

import sys
s = input()

for i in range(97, 123):
    st = chr(i)
    if st not in s:
        print(st)
        return

    
print(None)

import string

s = list(map(str, input()))
s = list(set(s))
s.sort()

ans = ""
arr = []

alp = list(map(str, string.ascii_lowercase))
for char in s:
    alp.remove(char)

if len(alp) == 0:
    print("None")
else:
    print((alp[0]))


import string
s = input()
t = set(s) ^ set(string.ascii_lowercase)

for i in string.ascii_lowercase:
  if set(i) & t:
    print(i)
    break
else:
  print("None")

S = str(input())
al = [chr(ord('a') + i) for i in range(26)]
data = []
for x in S:
    if not x in data:
        data.append(x)
ans = list(set(al) ^ set(data))
ans.sort()
if len(ans) == 0:
    print('None') 
else:
    print(ans[0])

s = list(input())
s.sort()
li =[]
lis = [97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112,
 113, 114, 115, 116, 117, 118, 119, 120, 121, 122]

for i in s:
    li.append(ord(i))
cnt = 0
for i in lis:
    if i in li:
        cnt += 1
    else:
        print((chr(i)))
        return
print('None')


A = ['a','s','d','f','g','h','j','k','l','z','x','c','v','b','n','m','q','w','e','r','t','y','u','i','o','p']
S = list(set(input()))
for i in S:
    A.remove(i)
A.sort()
try:
    print((A[0]))
except:
    print("None")


s = input()

alphabets = 'abcdefghijklmnopqrstuvwxyz'

for i in range(len(alphabets)):
   if not alphabets[i] in s:
      print(alphabets[i])
      return

print('None')

import sys
S = list(input())
s = list(set(S))
s.sort()
alfa = list('abcdefghijklmnopqrstuvwxyz')
ans = 0
if(len(s)==len(alfa)):
  print('None')
  return
else:
  for i in range(len(s)):
    if(s[i]!=alfa[i]):
      print(alfa[i])
      return
  print(alfa[len(s)])

import sys
import math
import itertools
import collections
import heapq
import re
import numpy as np
from functools import reduce

rr = lambda: sys.stdin.readline().rstrip()
rs = lambda: sys.stdin.readline().split()
ri = lambda: int(sys.stdin.readline())
rm = lambda: list(map(int, sys.stdin.readline().split()))
rl = lambda: list(map(int, sys.stdin.readline().split()))
inf = float('inf')
mod = 10**9 + 7

s = rr()
for char in 'abcdefghijklmnopqrstuvwxyz':
    if char not in s:
        print(char)
        return
else:
    print('None')














import math
from math import gcd,pi,sqrt
INF = float("inf")

import sys
sys.setrecursionlimit(10**6)
import itertools
from collections import Counter,deque
def i_input(): return int(input())
def i_map(): return list(map(int, input().split()))
def i_list(): return list(i_map())
def i_row(N): return [i_input() for _ in range(N)]
def i_row_list(N): return [i_list() for _ in range(N)]
def s_input(): return input()
def s_map(): return input().split()
def s_list(): return list(s_map())
def s_row(N): return [s_input for _ in range(N)]
def s_row_str(N): return [s_list() for _ in range(N)]
def s_row_list(N): return [list(s_input()) for _ in range(N)]

import string

def main():
    a  = string.ascii_lowercase
    s = s_input()
    for i in a:
        if i not in s:
            print(i)
            return
    print("None")


def __starting_point():
    main()

__starting_point()

s=input()
for i in range(ord("a"),ord("z")+1):
  if chr(i) not in s:
    print(chr(i)) 
    return
print("None")

from collections import Counter

S = list(input())
ac = Counter(S)

alp = 'abcdefghijklmnopqrstuvwxyz'

for a in alp:
    if not(a in ac):
        print(a)
        break
else:
    print('None')


# 35
import string

S = str(input())

set1 = set()
for s in S:
    set1.add(s)

set2 = set()
for alf in string.ascii_lowercase:
    set2.add(alf)

list = sorted(set2 - set1)
if not list: print('None')
else: print(sorted(list)[0])

S = input()
alphabets = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z']
ans = []
for alpha in alphabets:
    if S.count(alpha) > 0 :
        ans.append(1)
    else:
        ans.append(0)

if sum(ans) == 26:
    print('None')
else:
    print((alphabets[ans.index(0)]))


s = list(map(str,input()))
alp = "abcdefghijklmnopqrstuvwxyz"
s = set(s)
for i in range(26):
  if alp[i] not in s:
    print(alp[i])
    return
  elif i == 25:
    print("None")

s = input()

alphabets = 'abcdefghijklmnopqrstuvwxyz'

for x in alphabets:
    if x in s:
        continue
    else:
        print(x)
        break
else:
    print('None')

s = input()
ans = 'None'
line = [0 for i in range(26)]
for i in s:
  line[ord(i)-97] += 1
for j in range(26):
  if line[j] == 0:
    ans = chr(j + 97)
    break
print(ans)

li = list("abcdefghijknmlopqrstuvwxyz")
S = list(input())
ans = "None"
for l in li:
    if not l in S:
        ans = l
        break
print(ans)

s = input()
al = "abcdefghijklmnopqrstuvwxyz"
for i in al:
    if i not in s:
        print(i)
        break
else:
    print("None")

s = set(input())
a = "abcdefghijklmnopqrstuvwxyz"
for si in s:
    a = a.replace(si,"")
print(None if a=="" else a[0])

s = list(input())
import collections
import sys
a = collections.Counter(s)

for i in range(97,97+26):
    if chr(i) not in a:
        print(chr(i))
        return
else:
    print("None")

S = input()

import string
lst = list(string.ascii_lowercase)
d = {lst[i]:i for i in range(len(lst))}
cnt = [0]*len(lst)

for ch in S:
  cnt[d[ch]] = 1
  
idx = -1
for i in range(len(cnt)):
  if cnt[i] == 0:
    idx = i
    break

if idx == -1:
  print("None")
else:
  print((lst[idx]))


d = {}
for s in input():
  if s in d:
    d[s] += 1
  else:
    d[s] = 1

for a in 'abcdefghijklmnopqrstuvwxyz':
  if a not in d:
    print(a)
    return

print('None')

l = set("abcdefghijklmnopqrstuvwxyz")
s = set(input())
ans = sorted(list(l^s))
print("None" if len(ans)==0 else ans[0])

s = input()
s = set(s)
data = ['a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z']
for i in range(len(data)):
    if data[i] not in s:
        print(data[i])
        break
else:
     print('None')   

S=input()

alph="abcdefghijklmnopqrstuvwxyz"

for i in alph:
    if i in S:
        alph=alph.replace(i,"")

if len(alph)==0:
    print("None")
else:
    print(alph[0])

A = [0 for i in range(26)]
S = input()
for s in S:
  A[ord(s)-97] = 1
flag = False
for i in range(26):
  if A[i]==0:
    flag = True
    break
if flag:
  print(chr(97+i))
else:
  print("None")

import string

s = input()
for x in string.ascii_lowercase:
	if x not in s:
		print(x)
		break
else:
	print('None')

s=input()
res="None"
for i in range(ord("a"),ord("z")+1):
    if chr(i) not in s:
        res=chr(i)
        break
print(res)

s = input()
s = list(set(s))
s = sorted(s)
alf = [chr(i) for i in range(97,97+26)]
for i in range(26):
    if alf[i] not in s:
        print((alf[i]))
        return
        
print('None')


s=input()
f=1
for i in 'abcdefghijklmnopqrstuvwxyz':
    if i not in s:
        print(i)
        f=0
        return
if f:print('None')

S = list(input())
A = list('abcdefghijklmnopqrstuvwxyz')
ans = 'None'
for a in A:
    if a not in S:
        ans = a
        break
print(ans)

c = [chr(i) for i in range(97, 97+26)]
s = input()
ans = 'None'
for i in c:
    if i not in s:
        ans = i
        break
print(ans)

def main():
    s=input()
    for x in range(ord('a'), ord('z')+1):
        c=chr(x)
        if c in s:
            continue
        else:
            print(c)
            return
    print('None')
main()

S = sorted(list(set(input())))

alphabet = list('abcdefghijklmnopqrstuvwxyz')

for i in range(len(alphabet)):
    if alphabet[i] not in S:
        print(alphabet[i])
        return

print('None')

S=input()
a=[0]*26
for i in range(len(S)):
    tmp=ord(S[i])-ord('a')
    a[tmp]=1
    
for i in range(26):
    if a[i]==0:
        tmp=chr(ord('a')+i)
        print(tmp)
        break
else:
    print('None')


S = list(input())

S.sort()

if S[0] != 'a':
    print('a')
elif S[0] == 'a':
    for i in range(len(S)-1):
        if ord(S[i+1]) - ord(S[i]) == 0 or ord(S[i+1]) - ord(S[i]) == 1:
            pass
        else:
            res = ord(S[i]) + 1
            print((chr(res)))
            return
            
    res = ord(S[-1]) + 1

    if S[0] == 'a' and S[-1] == 'z':
        print('None')
    elif S[0] != 'a':
        print('a')
    else:
        print((chr(res)))


# -*- coding: utf-8 -*-

S = set(input())
az = [chr(i) for i in range(ord('a'),ord('z')+1)]

if len(S) >= 26:
    print("None")
    return

for i in az:
    if i in S:
        pass
    else:
        print(i)
        return





s = str(input())
L =['a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z']
for i in s:
  if i in L:
    L.remove(i)
if L == []:
  print('None')
else:
  print(L[0])

# -*- coding: utf-8 -*-

S = sorted(set(list(input())))

ans = 'None'
for keyword in list("abcdefghijklmnopqrstuvwxyz"):
    hit = False
    for s in S:
        if s == keyword:
            hit = True
            break 
    
    if hit == False:
        ans = keyword
        break

print(ans)



alphabet = [chr(i) for i in range(97, 123)]
S = set(list(input()))
for s in S:
  if s in alphabet: alphabet.remove(s)
    
print(alphabet[0]) if alphabet else print("None")

#from statistics import median
#import collections
#aa = collections.Counter(a) # list to list || .most_common(2)で最大の2個とりだせるお a[0][0]
from fractions import gcd
from itertools import combinations,permutations,accumulate, product # (string,3) 3回
#from collections import deque
from collections import deque,defaultdict,Counter
import decimal
import re
import math
import bisect
#
#
#
# pythonで無理なときは、pypyでやると正解するかも！！
#
#
# my_round_int = lambda x:np.round((x*2 + 1)//2)
# 四捨五入g
#
# インデックス系
# int min_y = max(0, i - 2), max_y = min(h - 1, i + 2);
# int min_x = max(0, j - 2), max_x = min(w - 1, j + 2);
#
#
import sys
sys.setrecursionlimit(10000000)
mod = 10**9 + 7
#mod = 9982443453
#mod = 998244353
from sys import stdin
readline = stdin.readline
def readInts():
  return list(map(int,readline().split()))
def readTuples():
    return tuple(map(int,readline().split()))
def I():
  return int(readline())
ans = "abcdefghijklmnopqrstuvwxyz"
s = input()
s = sorted(list(set(s)))
for v in ans:
    flag = True
    for j in range(len(s)):
        if s[j] == v:
            flag = False
    if flag:
        print(v)
        return
print(None)


S = input()

abc = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z']


from collections import defaultdict

d = defaultdict(int)
for s in S:
    d[s] = 1

for a in abc:
    if d[a] == 0:
        print(a)
        return
print('None')

s = input()
alphabets = [0] * 26

for i in s:
    alphabets[ord(i) - ord("a")] += 1

for i in range(26):
    if alphabets[i] == 0:
        print((chr(i + ord("a"))))
        break
else:
    print("None")


s = list(input())
flag = [0] * 26

for i in range(len(s)):
  flag[ord(s[i]) - 97] = 1

c = 0
for i in range(26):
  if flag[i] == 0:
    print(chr(i + 97))
    break
  else:
    c += 1
if c == 26:
  print("None")

S = input()
N = len(S)
for i in range(26):
    for k in range(N):
        if S[k] == chr(97+i):
            break
    else:
        print(chr(97+i))
        return
print('None')

S = input()
l = sorted(list(set(S)))

ans = ""
for i in range(len(l)):
  if ord(l[i]) !=  97 + i:
    ans = chr(97+i)
    break
  elif i == 25 and l[i] == "z":
    ans = "None"

if ans != "":
 print(ans)
else:
  print((chr(ord(l[-1])+1)))


alpha="abcdefghijklmnopqrstuvwxyz"
S=input()
for i in range(len(alpha)) :
  if alpha[i] not in S :
    print(alpha[i])
    return
print("None")

a=input()
s=[0]*26
for i in range(len(a)):
    s[ord(a[i])-97]=1
if sum(s)==26:
    print("None")
else:
    print(chr(s.index(min(s))+97))

s = set(input())

for i in range(26):
    if chr(i + ord("a")) not in s:
        print((chr(i + ord("a"))))
        break
else:
    print("None")


S = list(input())
S = list(set(S))

ans = "None"
for i in range(97, 123):
    cnt = chr(i)
    if not cnt in S:
        ans = cnt
        break
        
print(ans)

alpha = list('abcdefghijklmnopqrstuvwxyz')
s = input()
for i in alpha:
    if not i in s:
        print(i)
        return
print('None')

#-*-coding:utf-8-*-
import sys
input=sys.stdin.readline

def main():
    alpha_data={}
    S = input()
    [alpha_data.update({chr(i):0}) for i in range(97, 97+26)]
    
    for s in S:
        if s in alpha_data:
            tmp=alpha_data[s]
            tmp+=1
            alpha_data[s]=tmp
        else:
            continue
    for i,v in alpha_data.items():
        if v==0:
            print(i)
            return
    print("None")

def __starting_point():
    main()
__starting_point()

S = str(input())

no_ans = "None"
ans_list = []
arufabet = ["a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m", "n", "o", "p", "q", "r", "s", "t", "u", "v", "w", "x", "y", "z"]

for i in range(len(arufabet)):
    if arufabet[i] not in S:
        ans_list.append(arufabet[i])

if len(ans_list) == 0:
    print(no_ans)
else:
    print((ans_list[0]))


S = [s for s in input()]
li = list(set(S))
li.sort()

ans = 'None'
for c in [chr(c) for c in range(97, 123)]:
    if c not in li:
        ans = c
        break
print(ans)


S=input()
for i in "abcdefghijklmnopqrstuvwxyz":
  if not i in S:
    print(i)
    break
else:
  print("None")

s = input()
for i in range(97, 123):
  if chr(i) not in s:
    print(chr(i))
    break
else: print("None")

s = input()
alpha = ["a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z"]
l = list(set(alpha) - set(s))
l.sort()
print(l[0] if len(l) != 0 else "None")

s = set(input())
alp = 'abcdefghijklmnopqrstuvwxyz'
for a in alp:
    if a not in s:
        print(a)
        return
print('None')

s = input()
cnt = 0
for i in range(26):
    if chr(97 + i) in s:
        cnt += 1
        if cnt == 26:
            print("None")
            break
    else:
        print(chr(97+i))
        break

S = input()

a = [True for _ in range(26)]

for s in S:
    a[ord(s) - ord("a")] = False

for i, aa in enumerate(a):
    if aa:
        print((chr(i + ord("a"))))
        return

print("None")


s=input()
alphabet='abcdefghijklmnopqrstuvwxyz'
for i in alphabet:
  if i not in s:
    print(i)
    return
print('None')

S = list(input())
a = list("abcdefghijklmnopqrstuvwxyz")
for i in S:
    if i in a:
        a.remove(i)

if a:
    print(a[0])
else:
    print("None")

S = list(input())
L = len(S)
Slist = set()
for i in range(L):
    Slist.add(ord(S[i]))
Slist = list(Slist)
Slist.sort()
ans = "None"
j = 0
L = len(Slist)
for i in range(97, 123):
    if Slist[j] == i:
        j += 1
        if j >= L and i+1 < 123:
            ans = chr(i+1)
            break
        continue
    ans = chr(i)
    break
print(ans)


#from statistics import median
#import collections
#aa = collections.Counter(a) # list to list || .most_common(2)で最大の2個とりだせるお a[0][0]
from math import gcd
from itertools import combinations,permutations,accumulate, product # (string,3) 3回
#from collections import deque
from collections import deque,defaultdict,Counter
import decimal
import re
import math
import bisect
import heapq
#
#
#
# pythonで無理なときは、pypyでやると正解するかも！！
#
#
# my_round_int = lambda x:np.round((x*2 + 1)//2)
# 四捨五入g
#
# インデックス系
# int min_y = max(0, i - 2), max_y = min(h - 1, i + 2);
# int min_x = max(0, j - 2), max_x = min(w - 1, j + 2);
#
#
import sys
sys.setrecursionlimit(10000000)
mod = 10**9 + 7
#mod = 9982443453
#mod = 998244353
INF = float('inf')
from sys import stdin
readline = stdin.readline
def readInts():
  return list(map(int,readline().split()))
def readTuples():
    return tuple(map(int,readline().split()))
def I():
    return int(readline())
s = input()
lis = "abcdefghijklmnopqrstuvwxyz"
for i in range(len(lis)):
    if lis[i] in s:
        pass
    else:
        print((lis[i]))
        return
print('None')


S=input()
T="abcdefghijklmnopqrstuvwxyz"

for c in T:
    if S.count(c)==0:
        print(c)
        break;
else:
    print("None")


S = set(list(input()))
cf = [chr(ord("a")+i) for i in range(26)]

for i in range(len(cf)):
    if cf[i] not in S:
        print(cf[i])
        return
print("None")

import string

all_string = set(string.ascii_lowercase)
S = set(input())

ans = sorted(list(all_string - S))

print(ans[0]) if ans != [] else print("None")

from collections import Counter
S = list(input())
S = Counter(S)
if len(S) == 26:
    ans = "None"
else:
    A = [chr(i) for i in range(97, 97+26)]
    for i in A:
        if i in S:
            continue
        ans = i
        break
print(ans)


S=input()
X=set([s for s in 'abcdefghijklmnopqrstuvwxyz'])
for s in S:
  X.discard(s)
X=list(X)
X.sort()
X.append('None')
print(X[0])

s = input()
r = set(map(chr, range(97, 97+26)))
r = r - set(s)
if len(r) == 0:
    print('None')
else:
    r = min(r)
    print(r)

alphabets = [chr(ord('a') + i) for i in range(26)]

for a in list(input()):
    if a in alphabets:
        alphabets.remove(a)


if alphabets:
    print((alphabets[0]))
else:
    print("None")


from string import ascii_lowercase


def answer(s: str) -> str:
    not_in_s = list(set(ascii_lowercase) - set(s))
    if not not_in_s:
        return 'None'
    not_in_s.sort()

    return not_in_s[0]


def main():
    s = input()
    print(answer(s))


def __starting_point():
    main()
__starting_point()

import sys
alphabet = "abcdefghijklmnopqrstuvwxyz"
s = set(sorted(input().strip()))
for i in alphabet:
    if not i in s:
        print(i)
        return
print("None")




s = str(input())

alp = set("abcdefghijklmnopqrstuvwxyz")
s = set(s)
#print(s & alp)

if len(s & alp)==26:
    ans = "None"
else:
    lis = list(alp-s)
    lis.sort()
    ans = lis[0]

print(ans)


print("None" if len(s:=(set(list("abcdefghijklmnopqrstuvwxyz"))-set(input())))==0 else sorted(list(s))[0])

s = input()
d_l = [chr(i) for i in range(97, 97+26)]
ans = None
for d in d_l:
    if d not in list(s):
        ans = d
        break
print(ans)

print(min(set(map(chr,range(97,123)))-set(input())or['None']))

s = input()
alpabet = [chr(x) for x in range(97,123)]
try:
    for a in alpabet:
        s.index(a)
    else:
        print(None)
except ValueError as e:
    print(a)

import re
s = str(input())
l = list('abcdefghijklmnopqrstuvwxyz')

for i in l:
    if not re.search(i,s):
        print(i)
        break
else:
    print('None')

s = input()
alphabet = "abcdefghijklmnopqrstuvwxyz"

for i in alphabet:
    if i in s:
        alphabet = alphabet.replace(i,"")

if len(alphabet) == 0:
    print("None")
else:
    print(alphabet[0])

s = sorted(set(input()))
a = len(set(s))
s.append(0)
for i in range(26):
    if s[i] != chr(97+i):
        print(chr(97+i))
        break
else:
    print('None')

import string
S = set(input())
for i in range(97, 123):
  s = chr(i)
  if s not in S:
    print(s)
    break
else:
  print(None)

S = input()
for c in "abcdefghijklmnopqrstuvwxyz":
    if c not in S:
        print(c)
        break
else:
    print("None")


s = sorted(list(set(input())))

for i in range(26):
    if chr(ord("a") + i) not in s:
         print(chr(ord("a") + i))
         break
else:
    print("None")

s = input()
s = list(s)
s = set(s)
l = [chr(ord("a")+i) for i in range(26)]
ans = 'None'
for i in l:
    if i not in s:
        ans = i
        break
print(ans)

s = input()
alphabet = 'abcdefghijklmnopqrstuvwxyz'

for i in alphabet:
  if i not in s:
    print(i)
    return
print('None')

# -*- coding:utf-8 -*-
S = input()

alpahbet = "abcdefghijklmnopqrstuvwxyz"
string_list = []
ans = "None"
for s in S:
    if s not in string_list:
        string_list.append(s)

for a in alpahbet:
    if a not in string_list:
        ans = a
        break

print(ans)

s=input()

for i in "abcdefghijklmnopqrstuvwxyz":
  if i not in s:
    print(i)
    break
else:
  print("None")

S = list(input())
alpha = 'abcdefghijklmnopqrstuvwxyz'

for a in alpha:
  if S.count(a) == 0:
    print(a)
    break
else:
  print("None")

s = input()
c = "a"
while c <= "z":
    if s.find(c) == -1:
        print(c)
        break
    if c == "z":
        print("None")
    c = chr(ord(c) + 1)



import string
L = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z']
S = set(input())
for i in L:
  if i not in S:
    print(i)
    break
else:
  print(None)