Problem:
 Score : 200 points    Problem Statement 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 1 atcoderregularcontest      Sample Output 1 b  The string atcoderregularcontest contains a , but does not contain b .       Sample Input 2 abcdefghijklmnopqrstuvwxyz      Sample Output 2 None  This string contains every lowercase English letter.       Sample Input 3 fajsonlslfepbjtsaayxbymeskptcumtwrmkkinjxnnucagfrg      Sample Output 3 d

Solution (Python):
S = str(input())
a = [0] * 26


num2alpha = lambda c: chr(c+96)



alpha2num = lambda c: ord(c) - ord('a') + 1



for i in range(len(S)):
  x = alpha2num(S[i])
  a[x - 1] += 1
  
if min(a) != 0:
  print("None")
else:
  ans = a.index(0)
  print(num2alpha(ans + 1))

Solution (Python):
s = input()
for i in range(26):
    if s.count(chr(ord('a')+i)):
        continue
    else:
        print(chr(ord('a')+i))
        exit()
print('None')

Solution (Python):
import sys
sys.setrecursionlimit(4100000)
import math
import itertools
INF = 10**9

def main():
    S = input()
    alphabet = 'abcdefghijklmnopqrstuvwxyz'

    count = 0
    for s in alphabet:
        if s not in S:
            print(s)
            return
        if len(set(alphabet)- set(S)) == 0:
            print(None)
            return


if __name__ == '__main__':
    main()

Solution (Python):
import sys
sys.setrecursionlimit(4100000)
import math
import itertools
INF = 10**9

def main():
    S = input()
    alphabet = 'abcdefghijklmnopqrstuvwxyz'

    S_set = set(S)
    alphabet_set = set(alphabet)

    array = list(alphabet_set - S_set)
    array.sort()
    if len(array) == 0:
        print('None')
    else:
        print(array[0])


if __name__ == '__main__':
    main()

Solution (Python):
s = set(list(input()))
strings = [chr(i) for i in range(97, 97+26)]

for i in strings:
    if not i in s:
        print(i)
        exit()
print(None)

Solution (Python):
str1 ='abcdefghijklmnopqrstuvwxyz'
N = input()
count = 0
for i in str1:
  if i not in N:
    print(i)
    count +=1
    break
if count ==0:
  print('None')

Solution (Python):
s = input()
ans = 'None'
for i in range(26):
    c = chr(ord('a') + i)
    if c not in s:
        ans = c
        break
print(ans)

Solution (Python):
s = set(input())
a = set("abcdefghijklmnopqrstuvwxyz")
d = a - s
if d:
    print(sorted(d)[0])
else:
    print("None")

Solution (Python):
s = input()
a = "abcdefghijklmnopqrstuvwxyz"

for i in s:
    a = a.replace(i, "")
else:
    if len(a):
        print(a[0])
    else:
        print("None")

Solution (Python):
s = list(str(input()))
s.sort()
a = [chr(ord('a') + i) for i in range(26)]
b = {}
for i in a:
    b[i] = 0
for i in s:
    b[i] += 1

alf = ''
for i in a:
    if b[i] == 0:
        alf = i
        break
if alf:
    print(i)
else:
    print('None')

Solution (Python):
S = input()
alp = list('abcdefghijklmnopqrstuvwxyz')
for i in range(len(S)):
  if S[i] in alp:
    alp.remove(S[i])
if len(alp) == 0:
  print('None')
else:
  print(alp[0])

Solution (Python):
from sys import exit
S = input()
for letter in 'abcdefghijklmnopqrstuvwxyz':
  if letter not in S:
    print(letter)
    exit(0)
print(None)

Solution (Python):
s = list(input())

s.sort()

ans = "None"
for i in range(97,123):
    if s.count(chr(i)) == 0:
        ans = chr(i)
        break

print(ans)

Solution (Python):
p = input()

s = list(p)
s.sort()

answer = -1

check = []
for i in range(26):
    check.append(False)

for i in range(len(s)):
    x = ord(s[i])
    check[x - 97] = True

for i in range(26):
    if not check[i]:
        answer = i + 97
        break

if answer == -1:
    print('None')
else:
    print(chr(answer))

Solution (Python):
S = list(input())
USED = [False] * 26

for s in S:
  USED[ord(s)-97] = True
  
ok = False
ans = None
for i in range(26):
  if not USED[i]:
    ans = chr(97+i)
    ok = True
    break
    
if ok:
  print(ans)
else:
  print("None")

Solution (Python):
S = list(input())
chars = [chr(ord('a')+i) for i in range(26)]
for i in chars:
    if i not in S:
        print(i)
        exit()
print('None')

Solution (Python):
T = input()
L = [chr(l) for l in range(ord("a"), ord("z")+1)]
L = [l for l in L if l not in T]
print(min(L) if len(L)>0 else "None")

Solution (Python):
S = input()
l = len(S)
mozi = []
for i in range(l):
    mozi.append(S[i])
mozi = list(set(mozi))
C = [chr(i) for i in range(ord("a"),ord("z")+1)]

for i in mozi:
    for j in C:
        if i == j:
            C.remove(j)
if len(C) == 0:
    print("None")
else:
    print(C[0])

Solution (Python):
S = input()
for i in range(97,123):
  if chr(i) in set(S):
    continue
  print(chr(i))
  exit()
print("None")

Solution (Python):
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"]
for i in range(len(s)) :
    if s[i] in alpha :
        alpha.remove(s[i])
if alpha :
    print(alpha[0])
else :
    print("None")

Solution (Python):
S = input()
a = [s for s in "abcdefghijklmnopqrstuvwxyz"]

for s in list(set(S)):
    if(s in a):
        a.remove(s)
if(a == []):
    print("None")
else:
    print(a[0])

Solution (Python):
s = set(input())
al = set(list(map(chr, [i for i in range(97, 97+26)])))
print('None' if len(s)==len(al) else min(al-s))

Solution (Python):
from sys import exit

s = input()

for i in range(ord('a'), ord('z')+1):
    if chr(i) not in s:
        print(chr(i))
        exit()

print('None')

Solution (Python):
import sys
s = input()
for one in range(97,97+26):
    c = chr(one)
    if not c in s:
        print(c)
        sys.exit()
print("None")

Solution (Python):
s_set = set(input())
ord_a = ord("a")
ans = "None"
for i in range(26):
    c = chr(ord_a + i)
    if c not in s_set:
        ans = c
        break
print(ans)

Solution (Python):
N=list(input())
s=set(N)
ans=''.join(sorted(s))
all=[chr(i) for i in range(97,97+26)]
if ans=='abcdefghijklmnopqrstuvwxyz':
  print('None')
  exit()
for i in range(len(ans)):
  if ans[i]!=all[i]:
    print(all[i])
    exit()
  elif i==len(ans)-1 :
    print(all[len(ans)])
    exit()

Solution (Python):
import sys

S = set(input())
alpha = [chr(i) for i in range(ord('a'), ord('z')+1)]
for i in alpha:
    if not i in S:
        print(i)
        sys.exit()
print('None')

Solution (Python):
S = input()

appeared = [False] * 26
for c in S:
    appeared[ord(c) - ord('a')] = True

for i in range(26):
    if not appeared[i]:
        print(chr(ord('a') + i))
        exit(0)

print("None")

Solution (Python):
s = input()

words = [True]*26
for i in range(len(s)):
  words[ord(s[i])-97] = False
for i in range(26):
  if words[i]:
    print(chr(i+97))
    exit()
print(None)

Solution (Python):
import sys
S = list(input())
D = ['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 d in D:
    if not d in S:
        print(d)
        sys.exit()
print('None')

Solution (Python):
import sys
S = input()
C = [False] * 26
for s in S:
    C[ord(s) - ord('a')] = True
for i in range(26):
    if not C[i]:
        print(chr(i + ord('a')))
        sys.exit()
print('None')

Solution (Python):
s = set(list(input()))
n = set(list(map(chr,list(range(97,97+26)))))
print('None' if len(s)==len(n) else min(n-s))

Solution (Python):
import string

s = input()

t = string.ascii_lowercase

for i in t:
    if i not in s:
        print(i)
        exit()

print("None")

Solution (Python):

a = [0] * 26
s = input()
for i in s:
    a[ord(i) - 97] = 1
print('None'*(sum(a)==26) or chr(a.index(0) + 97))

Solution (Python):
s=input()
ans='None'
dic='abcdefghijklnmopqrstuvwxyz'
for i in range(26):
    if dic[i] not in s:
        ans=dic[i]
        break
print(ans)

Solution (Python):
A="abcdefghijklmnopqrstuvwxyz"
S=input()
S=sorted(set(S))

for i in range(len(S)):
	if S[i]!=A[i]:
		print(A[i])
		exit()
if len(S)<len(A):
	print(A[len(S)])
	exit()
print("None")

Solution (Python):
alp='abcdefghijklmnopqrstuvwxyz'
S=input()
a=[0 for i in range(26)]
for s in S:
    a[alp.index(s)]+=1
if a.count(0)>=1:
    print(alp[a.index(0)])
else:
    print("None")

Solution (Python):
import string
import sys
s = str(input())
low = string.ascii_lowercase
fin = ''
for i in range(len(low)):
    if low[i] not in s:
        print(low[i])
        sys.exit()
print('None')

Solution (Python):
import string
s = str(input())
low = string.ascii_lowercase
fin = ''
for i in range(len(low)):
    if low[i] not in s:
        fin += low[i]
if len(fin) == 0:
    print('None')
else:
    print(fin[0])

Solution (Python):
S = input()

for n in range(97,123):
  if chr(n) not in S:
    print(chr(n))
    exit()

print("None")

Solution (Python):
S = input()
L = [chr(i) for i in range(ord('a'), ord('z')+1)]

for l in L:
    if l not in S:
        print(l)
        exit(0)

print("None")

Solution (Python):
S = sorted(list(set(list(input()))))
alfabet = ["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"]
count = 0
for char in alfabet:
    if not(char in S):
        print(char)
        break
    count+=1
if count == 26:
    print("None")

Solution (Python):

arr = [chr(i) for i in range(97, 97+26)]
s = set([c for c in input()])

for c in arr:
    if not (c in s):
        print(c)
        break
else:
    print("None")

Solution (Python):
import sys
input=sys.stdin.readline

alp=[chr(i) for i in range(97, 97+26)]
s=input().rstrip()

for i in alp:
    if not i in s:
        print(i)
        exit()
print(None)

Solution (Python):
S = set(input())
for i in range(97, 123):
    c = chr(i)
    if c not in S:
        print(c)
        exit()
print('None')

Solution (Python):
s = input()
bkt = [0 for _ in range(26)]
for i in range(len(s)):
    bkt[ord(s[i])-97] = 1
print('None' if sum(bkt) == 26 else chr(bkt.index(0) + 97))

Solution (Python):
s = input()
alphabet = list(chr(ord("a")+i)for i in range(26))
a = [0]*26

for i in s:
    for j in range(26):
        if i == alphabet[j]:
            a[j] = 1

b = []
for i in range(26):
    if a[i] == 0:
        b.append(alphabet[i])

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

Solution (Python):
S = set(input())
s = list('abcdefghijklmnopqrstuvwxyz')
for i in s:
    if i not in S:
        print(i)
        exit()
print('None')

Solution (Python):
alp = 'abcdefghijklmnopqrstuvwxyz'

d = {c:False for c in alp}

s = open(0).read().rstrip()

for c in list(s):
    d[c] = True

isnone = True
for c,b in d.items():
    if not b:
        isnone = False
        print(c)
        break
if isnone:
    print('None')

Solution (Python):
S = input()
S = sorted(S)
alpha = [chr(i)for i in range(97,97+26)]
ans = "None"
for W in alpha:
    if not(W in S):
        ans = W
        break
print(ans)

Solution (Python):
s=input()
A=[False]*26
for i in range(len(s)):
    A[ord(s[i])-97]=True
for i in range(26):
    if A[i]==False:
        print(chr(i+97))
        break
else:
    print("None")

Solution (Python):
s = input()

dic = "abcdefghijklmnopqrstuvwxyz"
nf = sorted(set(dic) - set(s))
ans = nf[0] if len(nf) > 0 else "None"

print(ans)

Solution (Python):


S = list(str(input()))

chr_ord = 97

for i in range(26):
    if chr(chr_ord + i) not in S:
        print(chr(chr_ord + i))
        exit()

print('None')

Solution (Python):
S = sorted(list(input()))
alphabe = list('abcdefghijklmnopqrstuvwxyz')

for s in alphabe:
    if s in S:
        continue
    else:
        ans = s
        break
else:
    ans = 'None'
    
print(ans)

Solution (Python):


import copy
import random
import bisect 
import fractions 
import math
import sys
import collections
from decimal import Decimal 

mod = 10**9+7
sys.setrecursionlimit(mod) 

d = collections.deque()
def LI(): return list(map(int, sys.stdin.readline().split()))

S = str(input())
al = list("abcdefghijklmnopqrstuvwxyz")

for i in al:
    if i in S:
        pass
    else:
        print(i)
        break
else:
    print("None")

Solution (Python):
s=input()
count=0
for i in range(97,97+26):
    if chr(i) not in s:
        print(chr(i))
        count+=1
        break
    
if count==0:
    print("None")

Solution (Python):
S = input()
alpha = [chr(ord('a') + i) for i in range(26)]

for letter in alpha:
  if not letter in S:
    print(letter)
    exit()
print('None')

Solution (Python):
S = list(input())
lst = [chr(i) for i in range(97, 97+26)]
res = sorted(list(set(lst) - set(S)))

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

Solution (Python):
s = input()

alphabet_list = ['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(s)):
  if s[i] in alphabet_list:
    try:
      alphabet_list.remove(s[i])
    except ValueError:
      continue

if alphabet_list:
  print(alphabet_list[0])
else:
  print('None')

Solution (Python):
s = input()

b = [True] * 26
for c in s:
    b[ord(c) - ord('a')] = False

ans = 'None'
for i in range(26):
    if b[i]:
        ans = chr(ord('a') + i)
        break
print(ans)

Solution (Python):
s = input()
s = sorted(s)
s = set(s)
alpha = 'a'
flg = 0
for i in range(1,27):
    if (alpha not in s):
        flg = 1
        break
    else : alpha = chr(ord(alpha)+1)
if (flg): print(alpha)
else: print("None")

Solution (Python):
import string
a=string.ascii_lowercase

S=list(input())

for i in range(26):
    if a[i] not in S:
        ans=a[i]
        break
    if i==25:
        ans="None"
print(ans)

Solution (Python):
S = input()
s_list = [i for i in S]


eng_list = ['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']

dif = set(eng_list) - set(s_list)
if list(dif) == []:
    print('None')
else:print(sorted(list(dif))[0])

Solution (Python):
s = input()

words = [True]*26
for i in range(len(s)):
  words[ord(s[i])-97] = False

for i in range(26):
  if words[i]:
    print(chr(97+i))
    exit()
print(None)

Solution (Python):
def resolve():
    s = input()
    al = []
    for i in range(26):
        al.append(False)
    
    for c in s:
        ord_c = ord(c) - ord('a')
        al[ord_c] = True
    
    for i in range(26):
        if al[i] == False:
            print(chr(ord('a') + i))
            break
        if i == 25 and al[i] == True:
            print("None")
resolve()

Solution (Python):
s = input()
s = set(s)
abc = "abcdefghijklmnopqrstuvwxyz"
for i in abc:
  if i not in s:
    print(i)
    break
else:
  print("None")

Solution (Python):
import collections

s = input()
c = collections.Counter(s)

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

Solution (Python):
S=input()

U=set([chr(ord("a")+k) for k in range(26)])
T=set()
for s in S:
    T.add(s)

V=list(U-T)
V.sort()

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

Solution (Python):
s = input()
for i in range(26):
  if chr(ord('a') + i) not in s:
    print(chr(ord('a') + i))
    exit()
print('None')

Solution (Python):
S = sorted(set(input()))
A = 'abcdefghijklmnopqrstuvwxyz'

for a in A:
    if a not in S:
        print(a)
        exit()
print('None')

Solution (Python):
s=input()
alp="abcdefghijklmnopqrstuvwxyz"
ans="None"
for i in alp:
  exist=False
  if i in s:
    exist=True
  if not exist:
    ans=i
    break
print(ans)

Solution (Python):
import sys

s = sorted(input())
for c in list('abcdefghijklmnopqrstuvwxyz'):
    if not c in s:
        print(c)
        sys.exit()

print('None')

Solution (Python):
S = set(input())
base = ord('a')
ans = 'None'
for i in range(26):
    a = chr(base+i)
    if a in S:
        continue
    ans = a
    break
print(ans)

Solution (Python):
s=input()
S=[s[i] for i in range(len(s))]
characters=[0 for i in range(97,123)]
len(characters)
for i in range(len(S)):
  characters[ord(S[i])-97]=characters[ord(S[i])-97]+1
if 0 in characters:
  print(chr(characters.index(0)+97))
else:
  print('None')

Solution (Python):









import sys
import string
input = sys.stdin.readline

S = input()
All_string = list(string.ascii_lowercase)

for i in range(len(S)):
    if S[i] in All_string:
        n = All_string.index(S[i])
        All_string.pop(n)

if not All_string:
    print("None")
else:
    print(All_string[0])

Solution (Python):
import sys
s=input()
for i in range(ord("a"),ord("z")+1):
  if chr(i) not in s:
    print(chr(i))
    sys.exit()
print("None")

Solution (Python):
s=input()
al="abcdefghijklmnopqrstuvwxyz"
ans=""
d={}
for i in range(int(len(s))):
    if s[i] not in d:
        d[s[i]]=1
for i in range(26):
    if al[i] not in d:
        ans+=al[i]
        print(ans)
        exit()
print("None")

Solution (Python):
S = set(list(input()))
char = [chr(ord('a') + i) for i in range(26)]
for c in char:
    if c not in S:
        print(c)
        break
else:
    print('None')

Solution (Python):
s=input()
l=[0]*26

for i in range(len(s)):
    l[ord(s[i])-97]=1
i=0
while True:
    if l[i]==0:
        print(chr(i+97))
        break
    else:
        i+=1
    if i==26:
        print('None')
        break

Solution (Python):
S = set(list(input()))

A = set(list('abcdefghijklmnopqrstuvwxyz'))

ans = list(A-S)
if len(ans) == 0:
    print('None')
else:
    ans.sort()
    print(ans[0])

Solution (Python):
S=input()
dp=[0]*26
for i in S:
  dp[ord(i)-97]=1
for i in range(26):
  if dp[i]==0:
    print(chr(i+97))
    exit()
print('None')

Solution (Python):
S = list(input())
S_sort = set(sorted(S))
all_sort = {'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 = all_sort - S_sort
print('None' if len(ans) == 0  else sorted(list(ans))[0])

Solution (Python):
def main():
  s = input()
  flg=0
  for i in range(0,26):
    if chr(ord('a')+i) not in s:
      print(chr(ord('a')+i))
      flg=1
      break
  if flg ==0:
    print("None")
    
main()

Solution (Python):
s = input()
for i in range(97, 123):
    if s.find(chr(i)) == -1:
        print(chr(i))
        exit()
    
print('None')

Solution (Python):
s = input()
for c in 'abcdefghijklmnopqrstuvwxyz':
    if s.find(c) == -1:
        print(c)
        exit()
    
print('None')

Solution (Python):
import collections
import string

s = input()
s_count = collections.Counter(s)
s_list = list(s_count.keys())
ans = []

for alphabet in list(string.ascii_lowercase):
  if not alphabet in s_list:
    ans.append(alphabet)
if ans != []:
  print(ans[0])
else:
  print("None")

Solution (Python):
import sys
S = str(input())
if len(S) < 1 or len(S) > 10 ** 5:
    sys.exit()
if not S.islower():
    sys.exit()

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

for I in array_a_to_z:
    if I not in S:
        print(I)
        sys.exit()
print("None")

Solution (Python):
arb = 'abcdefghijklmnopqrstuvwxyz'
S = input()
s_S =sorted(set(arb)^set(S))
print('None' if len(s_S) == 0 else s_S[0])

Solution (Python):
arb = 'abcdefghijklmnopqrstuvwxyz'
S = input()

ans =''
for i in arb:
    if i not in S:
        ans += i

sort_ans = sorted(ans)
if len(ans)==0:
    print('None')
else:
    print(sort_ans[0])

Solution (Python):
s = input()
result = 'None'
for i in range(26):
    if chr(ord('a')+i) not in s:
        result = chr(ord('a')+i)
        break
print(result)

Solution (Python):
import sys
s = list(input())
for i in range(0,26):
  if not chr(97+i) in s:
    print(chr(97+i))
    sys.exit()
print("None")

Solution (Python):
S = input()

for a in "abcdefghijklmnopqrstuvwxyz":
  if a not in S:
    print(a)
    exit()

print("None")

Solution (Python):
s1 = sorted(input())
s2 = "abcdefghijklmnopqrstuvwxyz"

for s in s2:
  if s in s1:
    pass
  else:
    print(s)
    break
else:
  print("None")

Solution (Python):
S = input()
alphabet = [chr(i) for i in range(97, 97+26)]

for s in S:
    if s in alphabet:
        alphabet.remove(s)

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

Solution (Python):
S = input().strip()
a = [False]*26
for s in S:
    a[ord(s)-ord('a')] = True

ans = "None"
for i in range(len(a)):
    if not a[i]:
        ans = chr(i+ord('a'))
        break
print(ans)

Solution (Python):
s = input()
alphabet = [chr(i) for i in range(97, 97+26)]
tf = True
for i in alphabet:
  if not(i in s):
    print(i)
    tf = False
    break
if tf:
  print('None')

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

Solution (Python):
import collections,sys
S=list(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"]
c=collections.Counter(S)
for i in alpha:
    if c[i]==0:
        print(i)
        sys.exit()
print("None")

Solution (Python):
def resolve():
    a = "abcdefghijklmnopqrstuvwxyz"
    S = input()
    for ch in a:
        if ch not in S:
            print(ch)
            return
    print("None")

resolve()

Solution (Python):
s = str(input())
count = 0
a = 0
for i in range(ord("a"), ord("z")+1):
    if chr(i) in s:
        count += 1
    else:
        a = chr(i)
        break 
print(a if count != 26 else "None")

Solution (Python):
s = set(input())
ls = ["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"]
k = 0

for i in range(26):
    alp = ls[i]
    if alp not in s:
        k = 1
        break

if k == 1:
    print(alp)
else:
    print("None")

Solution (Python):
import string
s = list(input())
alpha = list(string.ascii_lowercase)
n = len(alpha)
ans = "None"
for i in range(n):
  if alpha[i] not in s:
    ans = alpha[i]
    break

print(ans)

Solution (Python):
str = input()

s = 'a'

for i in range(26):
    ord_str = ord(s) + i
    if str.count(chr(ord_str)) == 0:
        print(chr(ord_str))
        break
    elif i == 25:
        print("None")

Solution (Python):


from fractions import gcd

from collections import Counter, deque, defaultdict
from heapq import heappush, heappop, heappushpop, heapify, heapreplace, merge
from bisect import bisect_left, bisect_right, bisect, insort_left, insort_right, insort
from itertools import accumulate, product, permutations, combinations, combinations_with_replacement

import string
S = input()

c = Counter(S)

for i in string.ascii_lowercase:
    if(not c[i]):
        print(i)
        exit()
print("None")

Solution (Python):
S = list(input())
all = list('abcdefghijklmnopqrstuvwxyz')
ans = sorted(list(set(all) - set(S)))
print(ans[0] if len(ans) != 0 else 'None')

Solution (Python):
def i():
	return int(input())
def i2():
	return map(int,input().split())
def s():
	return str(input())
def l():
	return list(input())
def intl():
	return list(int(k) for k in input().split())

s = l()
alf = "abcdefghijklmnopqrstuvwxyz"
for i in range(len(s)):
	alf = alf.replace(s[i],"")
if alf == "":
	print("None")
else:
	print(alf[0])

Solution (Python):
def i():
	return int(input())
def i2():
	return map(int,input().split())
def s():
	return str(input())
def l():
	return list(input())
def intl():
	return list(int(k) for k in input().split())

s = l()
alf = "abcdefghijklmnopqrstuvwxyz"
for i in range(len(s)):
	alf = alf.replace(s[i],'')
if alf == "":
	print("None")
else:
	print(alf[0])

Solution (Python):



S = input().strip()
alphabet = "abcdefghijklmnopqrstuvwxyz"

for ch in alphabet:
    if ch not in S:
        print(ch)
        exit()
print("None")

Solution (Python):
alphabet = 'abcdefghijklmnopqrstuvwxyz'
s = input()

for i in alphabet:
  if i not in s:
    print(i)
    exit()
print("None")

Solution (Python):
s = str(input())
apple = "abcdefghijklmnopqrstuvwxyz"
count = 0
a = 0
for i in apple:
    if i in s:
        count += 1
    else:
        a = i
        break
print(a if count != 26  else "None")

Solution (Python):
import string
a = string.ascii_lowercase
S =input()
a = sorted(set(a)^set(S))
if a:
    print(a[0])
else:
    print("None")

Solution (Python):
S = input()
lis_S = list(S)

a = "abcdefghijklmnopqrstuvwxyz"

for i in lis_S:
    a = a.replace(i,"")

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

Solution (Python):
S = input()
lll = "abcdefghijklmnopqrstuvwxyz"
for i in range(26):
    if lll[i] in S:
        pass
    else:
        print(lll[i])
        exit()
    
print("None")

Solution (Python):
S = input()
list_Alpha = list("qwertyuiopasdfghjklzxcvbnm")
list_S = list(S)
for i in list_S:
  if i in list_Alpha:
    list_Alpha.remove(i)
  else:
    continue
if bool(list_Alpha) == True:
  list_Alpha.sort()
  print(list_Alpha[0])
else:
  print("None")

Solution (Python):
S=str(input())

import string
alphabet="".join([i for i in string.ascii_lowercase if i not in S])

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

Solution (Python):
import sys

class alphabet: 
    def __init__(self, capitalize):
        self.num = dict() 
        self.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"]
        if capitalize: 
            for i in range(26): self.abc[i] = self.abc[i].upper()
        for i, a in enumerate(self.abc): self.num[a] = True


def solve():
    S = input()
    AB = alphabet(False)
    for i in range(len(S)):
        AB.num[S[i]] = False
    for a in AB.abc:
        if AB.num[a]:
            print(a)
            break
    else: print("None")

    return 0

if __name__ == "__main__":
    solve()

Solution (Python):
s = list(input())
exist = 'abcdefghijklmnopqrstuvwxyz'

j = 0
s = set(s)
s = sorted(s)
if len(s) == 26:
    print('None')
    exit()
for i in range(len(s)+1):
    try:
        if s[i] == exist[j]:
            pass    
        elif s[i] != exist[j]:
            print(exist[j])
            break
        j += 1
    except:
        print('z')

Solution (Python):
s = str(input())
a = "abcdefghijklmnopqrstuvwxyz"
count = 0
n = 0
for i in range(len(a)):
    for j in range(len(s)):
        if a[i] == s[j]:
            count += 1
            break
    if count == 0:
        ans = a[i]
        print(ans)
        n = 1
        break
    else:
        count = 0
if n == 0:
    print("None")

Solution (Python):
import string
S = set(input())
ascii_lowercase = 'abcdefghijklmnopqrstuvwxyz'
for al in ascii_lowercase:
    if al not in S:
        print(al)
        break
else:
    print('None')

Solution (Python):
import string
word=sorted(set((input()))^set(string.ascii_lowercase))
print(word and word[0] or 'None')

Solution (Python):
S=input()

if "a" not in S:
    print("a")
elif "b"  not in S:
    print("b")
elif "c"  not in S:
    print("c")
elif "d"  not in S:
    print("d")
elif "e"  not in S:
    print("e")
elif "f"  not in S:
    print("f")
elif "g"  not in S:
    print("g")
elif "h"  not in S:
    print("h")
elif "i"  not in S:
    print("i")
elif "j"  not in S:
    print("j")
elif "k"  not in S:
    print("k")
elif "l"  not in S:
    print("l")
elif "m"  not in S:
    print("m")
elif "n"  not in S:
    print("n")
elif "o"  not in S:
    print("o")
elif "p"  not in S:
    print("p")
elif "q"  not in S:
    print("q")
elif "r"  not in S:
    print("r")
elif "s"  not in S:
    print("s")
elif "t"  not in S:
    print("t")
elif "u"  not in S:
    print("u")
elif "v"  not in S:
    print("v")
elif "w"  not in S:
    print("w")
elif "x"  not in S:
    print("x")
elif "y"  not in S:
    print("y")
elif "z"  not in S:
    print("z")
else:
    print("None")

Solution (Python):


import sys
input=sys.stdin.readline

def main():
    ab="abcdefghijklmnopqrstuvwxyz"
    S=input()
    ans="None"
    for a in ab:
        if not a in S:
            ans=a
            break
    print(ans)
    
if __name__=="__main__":
    main()

Solution (Python):
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 = input()
for i in l:
    flag = False
    for aa in s:
        if aa == i:
            flag = True
    if flag == False:
        print(i)
        exit()
print("None")

Solution (Python):

s = list(input())
s = set(s)
s = sorted(s)
a = list(str('abcdefghijklmnopqrstuvwyz'))
for i in range(len(s)):
  if s[i] in a:
    a.remove(s[i])
if len(a) == 0:
  print('None')
else:
  print(a[0])

Solution (Python):
a=sorted(set(input())^set(chr(i)for i in range(97,123)));print(a and a[0]or None)

Solution (Python):
S = set(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"}

a = alphabet.difference(S)

ans = list(a)
ans.sort()

try:
    print(ans[0])
except:
    print("None")

Solution (Python):
s = list(input())
s = set(s)
s = sorted(s)
m = 'abcdefghijklmnopqrstuvwxyz'

for i in range(len(m)):
    if s[i] != m[i]:
        print(m[i])
        exit(0)

    if i + 1 != len(m) and i + 1 == len(s):
        print(m[i + 1])
        exit(0)

print('None')

Solution (Python):
s = input()
for i in range(ord('a'), ord('z')+1):
    if chr(i) in s:
        continue
    else:
        print(chr(i))
        break
else:
    print('None')

Solution (Python):
S=input()
for i in range(ord("a"),ord("z")+1):
  if chr(i) in S:
    continue
  else:
    print(chr(i))
    exit()
print("None")

Solution (Python):
x='abcdefghijklmnopqrstuvwxyz'
xs=set(x)
y=input()
ys=set(y)
d=xs-ys
d=sorted(list(d))
if d==[]:
    print('None')
else:
    print(d[0])

Solution (Python):
def resolve():
    S = input()
    abc = "abcdefghijklmnopqrstuvwxyz"
    for i in range(26):
        if S.count(abc[i]) == 0:
            print(abc[i])
            exit()
        else:
            continue
    print("None")
resolve()

Solution (Python):
s = input()
letters = set([chr(i) for i in range(97, 97+26)]) - set(s)
if  letters == set():
    print('None')
else:
    print(list(sorted(letters))[0])

Solution (Python):
import sys
input = sys.stdin.readline


def read():
    S = input().strip()
    return S,


def solve(S):
    for z in "abcdefghijklmnopqrstuvwxyz":
        if z not in S:
            return z
    return "None"


if __name__ == '__main__':
    inputs = read()
    print(solve(*inputs))

Solution (Python):
S = input()
for s in [chr(ord("a")+i) for i in range(26)]:
  if not s in S:
    print(s)
    exit()
    
print("None")

Solution (Python):
def searchstring(s) : 
  start = ord('a')
  end = ord('z')
  stringlist = [0] * 256
  for i in range(len(s)) : 
    stringlist[ord(s[i])] += 1
  for i in range(start, end + 1) : 
    if stringlist[i] == 0 : 
      return chr(i)
  return None



s = input()

print(searchstring(s))

Solution (Python):
S = input()

al = "abcdefghijklmnopqrstuvwxyz"
for a in al:
    if a not in S:
        print(a)
        break
else:
    print("None")

Solution (Python):
S = input()
S = sorted(list(set(S)))

n = 97
for i in range(len(S)):
    if ord(S[i]) == n:
        n += 1
    else:
        break

if n == 123:
    print('None')
else:
    print(chr(n))

Solution (Python):
S = input()
i = ord("a")

flag = 0

for i in range(i, i+ 26):
  if(chr(i) not in S):
    print(chr(i))
    flag = 1
    break
    
if(flag == 0):
  print("None")

Solution (Python):
s=set(input())
w="abcdefghijklmnopqrstuvwxyz"
ans=sorted(set(w)^s)
print("None" if len(ans)==0 else ans[0])