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 = 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))

Solution (Python):


import numpy as np

s=input()

for i in range(26):
    k=chr(ord('a') + i)
    
    if(not k in s):
        print(k)
        exit(0)

print("None")

Solution (Python):
S = input()


bucket = [False for _ in range(26)]

for i in S:
    bucket[ord(i)-ord('a')] = True

false_idx = -1

for i in range(26):
    if bucket[i] == False:
        false_idx = i
        break


if false_idx == -1:
    print('None')
else:
    print(chr(ord('a')+i))

Solution (Python):
S = set(input())
candidates = set('abcdefghijklmnopqrstuvwxyz')

if len(S) == 26:
    print('None')
else:
    print(sorted(list(candidates.difference(S)))[0])

Solution (Python):
s=input()
for i in range(ord('a'), ord('z')+1):
  if chr(i) not in s:
    print(chr(i))
    break
  elif chr(i)=='z':
    print('None')

Solution (Python):
input = input()

arr = {}

for i in range(ord('a'), ord('z') + 1):
	arr[i] = 0;

for s in input:
	arr[ord(s)] = 1

flag = True
for i in arr:
	if arr[i] == 0:
		print(chr(i))
		flag = False
		break
	
if flag:
	print('None')

Solution (Python):
import re

S=input()

char = [chr(i) for i in range(97, 97+26)]
ans = ""

for c in char:
  result = re.findall(c, S, re.S)

  if len(result) == 0:
    ans = c
    break
else:
  ans = "None"

print(ans)

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

ref = [chr(i) for i in range(97, 97+26)]

diff = list(set(s) ^ set(ref))
diff.sort()

if (len(diff) == 0):
   print('None')
else:
   print(diff[0])

Solution (Python):
s=input()
lis=["q","w","e","r","t","y","u","i","o","p","l","k","j","h","g","f","d","s","a","z","x","c","v","b","n","m"]
a=set()
for i in range(len(s)):
    a.add(s[i])

ans=[]
for i in range(len(lis)):
    if lis[i] in a:
        continue
    ans.append(lis[i])
ans.sort()
if ans:
    print(ans[0])
else:
    print("None")

Solution (Python):
s = input()
for l in [chr(i) for i in range(97, 97+26)]:
  if l not in s:
    print(l)
    exit()
print('None')

Solution (Python):
s = input()
_s = [a for a in s]
ss = set(_s)

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

diff = list(az - ss)
diff.sort()

if len(diff):
    print(diff[0])
else:
    print('None')

Solution (Python):
import sys, re
from collections import deque, defaultdict, Counter
from math import ceil, sqrt, hypot, factorial, pi, sin, cos, radians
from itertools import permutations, combinations, product, accumulate
from operator import itemgetter, mul
from copy import deepcopy
from string import ascii_lowercase, ascii_uppercase, digits
from fractions import gcd
from bisect import bisect

def input(): return sys.stdin.readline().strip()
def INT(): return int(input())
def MAP(): return map(int, input().split())
def LIST(): return list(map(int, input().split()))
sys.setrecursionlimit(10 ** 9)
INF = float('inf')
mod = 10 ** 9 + 7

S = input()

for c in ascii_lowercase:
	if c not in S:
		print(c)
		break
else:
	print("None")

Solution (Python):


s=input()
a=[chr(i) for i in range(97,97+26)]
flag=False
for i in range(26):
    if a[i] not in s:
        ans=i
        flag=True
    if flag:
        break
if flag:
    print(chr(97+i))
else:
    print("None")

Solution (Python):
import sys
input = sys.stdin.readline
S = input().rstrip()
al = [chr(i) for i in range(97, 97+26)]
for i in al:
    if i not in S:
        print(i)
        sys.exit()
print("None")

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

ans = ''
for x in X:
    if x not in S:
        ans = x
        break
if ans:
    print(ans[0])
else:
    print('None')

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

ans = ''
for x in X:
    if x not in S:
        ans = x
        break
if ans:
    print(ans)
else:
    print('None')

Solution (Python):
S = input()

X = ['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 x in X:
    if x not in S:
        ans = x
        break
if ans:
    print(ans[0])
else:
    print('None')

Solution (Python):
S = input()

X= ['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 x in X:
    if x not in S:
        ans = x
        break

if ans == '':
    ans = 'None'
print(ans)

Solution (Python):
s = input()
all_lst = [chr(ord('a') + i) for i in range(26)]
if len(set(s)) == 26:
    print('None')
else:
    diff = set(all_lst) - set(s)
    print(sorted(diff)[0])

Solution (Python):
alphabet = "abcdefghijklmnopqrstuvwxyz"
str = input()
dic = {}
for i in range(len(str)):
  if not str[i] in dic:
    dic[str[i]] = True
if len(dic) == 26:
  print("None")
else:
  for i in range(len(alphabet)):
    if alphabet[i] not in dic:
      print(alphabet[i])
      break

Solution (Python):
s = input()

res = "abcdefghijklmnopqrstuvwxyz"

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

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

Solution (Python):
S = list(set(list(input())))
S.sort()
al = 'abcdefghijklmnopqrstuvwxyz'
if len(S)==26:
  ans = 'None'
else:
  for i in range(len(S)):
    if al[i] != S[i]:
      ans = al[i]
      break
    else:
      ans = al[len(S)]
print(ans)

Solution (Python):
import sys


def solve(inp):
    S = inp.readline()

    alphas = {}
    for c in S:
        if c not in alphas:
            alphas[ord(c)] = 0
        alphas[ord(c)] += 1

    for c in range(ord('a'), ord('z') + 1):
        if c not in alphas:
            return chr(c)

    return "None"


def main():
    result = solve(sys.stdin)
    if result:
        print(result)


if __name__ == '__main__':
    main()

Solution (Python):
S = input()
mylist=([chr(ord('a') + i) for i in range(26)])
for i in range(26):
    if mylist[0] in S:
        mylist.pop(0)
        pass
    else:
        print(mylist[0])
        break
if mylist==[]:
    print("None")

Solution (Python):


s = input()

lis = set(list(s))

dic = ["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 lis:
    dic.remove(i)

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

Solution (Python):
S = input()
origin = set()
for i in range(26):
    origin.add(chr(ord('a') + i))

s = set(S)
li = list(origin - s)
if len(li) == 0:
    print('None')
else:
    print(sorted(li)[0])

Solution (Python):
s = input()
alp = [chr(i) for i in range(ord('a'),ord('z')+1)]
ans = []
for i in alp:
    if i not in s:
        ans.append(i)
ans.sort()
if ans:
    print(ans[0])
else:
    print('None')

Solution (Python):
S = input().strip()
alph = [chr(i) for i in range(97, 97+26)]
for i in S:
  if i in alph:
    alph.remove(i)
if alph:
  print(alph[0])
else:
  print("None")

Solution (Python):
s=list(set(input()))
alph=[chr(i) for i in range(97,97+26)]
for i in s:
    if i in alph:
        alph.remove(i)
if alph:
    print(alph[0])
else:
    print("None")

Solution (Python):
s=input()
alph=[chr(i) for i in range(97,97+26)]
for i in s:
    if i in alph:
        alph.remove(i)
if alph:
    print(alph[0])
else:
    print("None")

Solution (Python):
s = str(input())
alp = list('abcdefghijklmnopqrstuvwxyz')
lst = []

for i in range(len(s)):
    if s[i] in alp:
        alp.remove(s[i])

if alp==[]:
    print('None')
else:
    alp.sort()
    print(alp[0])

Solution (Python):
s=input()
flg=True
for i in range(97,97+26):
  if not chr(i) in s:
    print(chr(i))
    flg=False
    break
if flg:
  print("None")

Solution (Python):
s = str(input())
alphabet = list('abcdefghijklmnopqrstuvwxyz')
for i in alphabet:
    if i in s:
        continue
    else:
        print(i)
        exit(0)
print('None')

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

Solution (Python):
S=input()


for i in range(26):
    if chr(i+97) not in S:
        ans=chr(i+97)
        break
    else:
        ans="!"
        
if ans=="!":
    print("None")
else:
    print(ans)

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

l = []

for i in range(26):
    l.append(chr(i+ord("a")))

for i in range(len(S)):
    l.remove(S[i])

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

Solution (Python):
s=list(input())
a=[chr(i) for i in range(97,97+26)]
x=0
for i in range(26):
    if s.count(a[i])==0:
        print(a[i])
        x+=1
        break
if x==0:
    print('None')

Solution (Python):
s=input()

for i in range(97, 122+1):
    if chr(i) not in s:
        print(chr(i))
        break
    if chr(i) =="z":
        print("None")
        break

Solution (Python):
import sys
from collections import defaultdict, deque
import bisect
from heapq import *
from math import factorial
sys.setrecursionlimit(200000)
input = sys.stdin.readline







S = input().strip()

l = set([v for v in range(97, 123)])
for v in S:
    l = l - {ord(v)}

if len(l) == 0:
    print("None")
    exit()
print(chr(min(l)))

Solution (Python):

 
import math
import itertools
import sys
import copy
 




S = list(str(input()))
S.sort()
Q = "abcdefghijklmnopqrstuvwxyz"


index = 0
if S[0] == Q[0] :
  index += 1
else :
  print ("a")
  exit()

for i in range(1, len(S)) :
  if index == 26 :
    print ("None")
    exit()
  
  if S[i] == Q[index] :
    index += 1
  else :
    if S[i] == S[i-1] :
      continue
    print (Q[index])
    exit()


if index != 26 :
  print (Q[index])
else :
  print ("None")

Solution (Python):
import sys

for i in sys.stdin:
  t = [True for j in range(26)]
  for j in i.strip('\n'):
    t[ord(j) - 97] = False
  for j in range(len(t)):
    if t[j]:
      print(chr(j + 97))
      break
  else:
    print('None')

Solution (Python):
import string

s = input()
alp = string.ascii_lowercase

for c in s:
    if c in alp:
        alp = alp.replace(c, '', -1)

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

Solution (Python):
import string

S = input()

for s in string.ascii_lowercase:
    if s not in S:
        print(s)
        exit()

print("None")

Solution (Python):
k=set("aqwertyuioplkjhgfdsazxcvbnm")
s=set(input())
ks=list(k-s)
if len(ks)==0:
  print("None")
else:
  ks.sort()
  print(ks[0])

Solution (Python):
import itertools
s = sorted(set(input()))

ans = "None"
for i, _s in itertools.zip_longest([chr(i) for i in range(97, 97+26)], s):
    if i != _s:
        ans = i
        break

print(ans)

Solution (Python):
x=[chr(i) for i in range(97, 97+26)]
y=[0]*26
s=input()
for i in s:
    y[x.index(i)]=1
for i in range(26):
    if y[i]==0:
        print(chr(97+i))
        break
else:
    print("None")

Solution (Python):
s = input()

letters = list(map(chr, range(97, 123)))
ans = sorted(set(letters) - set(s))

print(ans[0] if ans else 'None')

Solution (Python):
S = str(input())
a=['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 a:
    a.remove(S[i])
print(None if a==[] else a[0])

Solution (Python):
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']

for i in range(len(ABC)):
 if ABC[i] not in S:
  print(ABC[i])
  exit()
else:
  print("None")

Solution (Python):
S = set(input())
alp = "abcdefghijklmnopqrstuvwxyz"
ans = [i for i in alp if not i in S]
if len(ans) == 0:
    print("None")
else:
    print(ans[0])

Solution (Python):
S = input()

a_to_z = "abcdefghijklmnopqrstuvwxyz"
ans = [0 for i in range(26)]
for i in range(26):
    if a_to_z[i] not in S:
        ans[i] = 1
    else:
        ans[i] = 0
        
if sum(ans[x] for x in range(26)) > 0:
    print(a_to_z[ans.index(1)])
else:
    print("None")

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

S = set(S)
alpha_list = set(alpha_list)
answer = alpha_list - S
if alpha_list == S:
    print('None')
else:
    print(sorted(list(answer))[0])

Solution (Python):
alp = {i for i in "abcdefghijkmlnopqrstuvwxyz"}
s = set(input())

l = sorted(list(alp - s))
if l:
    print(l[0])
else:
    print("None")

Solution (Python):
import sys
from string import ascii_lowercase

s = sys.stdin.readline().rstrip()

def main():
    res = sorted(set(ascii_lowercase) - set(s))
    return res[0] if res else None

if __name__ == '__main__':
    ans = main()
    print(ans)

Solution (Python):
S = input()

words = set(list(S))

alphabets = set(list("abcdefghijklmnopqrstuvwxyz"))


or_set = sorted(alphabets - words)
if or_set:
    print(or_set[0])
else:
    print("None")

Solution (Python):
S = input()

bucket = [0] * 26


def c_to_num(c):
    return ord(c) - 97


def num_to_c(num):
    return chr(num+97)


for c in S:
    bucket[c_to_num(c)] += 1

ret = 'None'
for i, n in enumerate(bucket):
    if not n:
        ret = num_to_c(i)
        break

print(ret)

Solution (Python):
s = set(input())
l = set(chr(i) for i in range(ord("a"), ord("z") + 1))

for i in sorted(l):
    if not i in s:
        print(i)
        break
    if i == "z":
        print("None")

Solution (Python):
s = input()
alpha = [chr(ord('a') + i) for i in range(26)]
for letter in s:
    if(alpha.count(letter)): alpha.remove(letter)
print(alpha[0]) if alpha else print('None')

Solution (Python):
li = list("abcdefghijklmnopqrstuvwxyz")
s = list(input())
cnt = [0 for i in range(26)]


for i in range(len(s)):
    for j in range(26):
        if li[j]==s[i]:
            cnt[j]+=1

for i in range(26):
    if cnt[i]==0:
        print(li[i])
        break
    elif i ==25:
        print("None")

Solution (Python):
s = input()

al = [chr(ord('a') + i) for i in range(26)]
al.sort()
s = set(s)
ans = 'None'
for a in al:
    if a not in s:
        ans = a
        break
print(ans)

Solution (Python):

s = input()
letter  = 'abcdefghijklmnopqrstuvwxyz'
s_sort = sorted(s)


for l in letter:
    if l not in s_sort:
        print(l)
        exit()

print("None")

Solution (Python):
s = list(input())
alp = list("abcdefghijklmnopqrstuvwsyz")
for i in alp:
    if i not in s:
        print(i)
        exit()
print('None')

Solution (Python):
s=list(input())
dic=[]
for i in range(26):
    dic.append([chr(i+97),0])

num=lambda x:ord(x)-ord("a")
for i in range(len(s)):
    if dic[num(s[i])][1]==0:
        dic[num(s[i])][1]=1
exit=0
ans=0
for i in range(26):
    if dic[i][1]==0:
        exit=1
        ans=i
        break
print(dic[i][0] if exit==1 else "None")

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

Solution (Python):
s=input()
alp="abcdefghijeklmnopqrstuvwxyz"
for i in alp:
    if i not in s:
        print(i)
        exit()
print("None")

Solution (Python):
import collections
dd = collections.defaultdict(bool)
s = input()
for i in s:
    dd[i] = True
for i in range(ord('a'), ord('z') + 1):
    if not dd[chr(i)]:
        print(chr(i))
        exit()
print('None')

Solution (Python):
alpha = {i.lower() for i in 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'}
s = set(input())

l = sorted(list(alpha - s))

print(l[0] if l else 'None')

Solution (Python):
import string

s = str(input())

s_set = set(s)
alpha_set = set(string.ascii_lowercase)

res = list(alpha_set - s_set)

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

print(ans)

Solution (Python):
s=input()
ls=list("abcdefghijklmnopqrstuvwxyz")
ans = "None"
for i in ls:
  if i not in s:
    ans = i
    break
print(ans)

Solution (Python):
import string
s=input()
for alpha in string.ascii_lowercase:
    if(alpha not in s):
        print(alpha)
        exit()
print('None')

Solution (Python):
s=set(input())
al=set([chr(ord('a') + i) for i in range(26)])
ans=list(al-s)
if ans!=[]:
	ans.sort()
	print(ans[0])
else:print('None')

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

for c in ch:
    if  c not in s:
        print(c)
        import sys
        sys.exit()
print("None")

Solution (Python):
x=[chr(i) for i in range(97, 97+26)]
s=input()
for i in x:
    if i not in s:
        print(i)
        break
else:
    print("None")

Solution (Python):
line = 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"]
letters = []
for i in range(len(line)):
  letters.append(line[i])
setted = sorted(set(letters))
for j in alpha[:]:
  for k in setted:
    if j == k:
      alpha.remove(j)
if len(alpha) == 0:
  print("None")
else:
  answer = alpha[0]
  print(answer)

Solution (Python):
Alph = "abcdefghijklmnopqrstuvwxyz"
S = list(input())
S.sort()
ans = "None"
for i in Alph:
        if i not in S:
                ans = i 
                break
print(ans)

Solution (Python):
print((sorted(list({chr(i) for i in range(ord('a'),ord('z')+1)}-set(input())))+[None])[0])

Solution (Python):
S = input()

hoge = ['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']

hoge2 = list(S)

differlen = len((set(hoge) ^ set(hoge2)))

differ2 = set(hoge) ^ set(hoge2)
differ3 = sorted(differ2)




if differlen == 0:
    print("None")
else:
    print(differ3[0])

Solution (Python):
S = sorted(list(input()))
S = list(map(lambda x: ord(x) - ord('a'), S))
S = list(set(S))

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

Solution (Python):

def main()->None:
    alphabet = "abcdefghijklmnopqrstuvwxyz"
    S = input()
    sorted_s = sorted(S)
    flag = False
    for i in alphabet:
        if i not in sorted_s:
            flag = True
            break
    if flag:print(i)
    else:print("None")



if __name__ == "__main__":
    main()

Solution (Python):
s = sorted(set(input()))
t = "abcdefghijklmnopqrstuvwxyz"
ans = "None"
for tt in t:
  if not tt in s:
    ans = tt
    break
print(ans)

Solution (Python):
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 i in alphabet:
    if i not in s:
        print(i)
        break
else:
    print("None")

Solution (Python):
s = sorted(input())
characters = ['a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','t','u','v','w','x','y','z']
ans = "None"
for i in characters:
      if i not in s:
        ans = i
        break
print(ans)

Solution (Python):
s=input()
l=[]
alf=["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 x in alf:
    if str(x) in s:
        pass
    else:
        l.append(x)
if l==[]:
    print("None")
else:
    print(min(l))

Solution (Python):
print((list(sorted(set(chr(ord("a") + i) for i in range(26)) - set(input()))) + ["None"])[0])

Solution (Python):
[print(un_appearence[0] if un_appearence else "None") for un_appearence in [[chr(i) for i in range(ord('a'), ord('z') + 1) if chr(i) not in string] for string in [input()]]]

Solution (Python):
from collections import Counter
S = list(input())
S = Counter(S)
S = list(S.keys())
S.sort()
for i in range(26):
    alph = chr(ord("a") + i)
    
    if i == len(S):
        print(chr(ord("a") + i))
        exit()
    elif S[i] != chr(ord("a") + i):
        print(alph)
        exit()

print("None")

Solution (Python):
s = input()

ls_small = [chr(_) for _ in range(97, 97+26)]

for i in range(len(ls_small)):
    if ls_small[i] not in s:
        print(ls_small[i])
        break
    elif i == len(ls_small) - 1:
        print("None")

Solution (Python):
import sys

sys.setrecursionlimit(10 ** 6)
int1 = lambda x: int(x) - 1
p2D = lambda x: print(*x, sep="\n")
def MI(): return map(int, sys.stdin.readline().split())
def LI(): return list(map(int, sys.stdin.readline().split()))
def LLI(rows_number): return [LI() for _ in range(rows_number)]

def main():
    find=[False]*26
    orda=ord("a")
    for c in input():
        find[ord(c)-orda]=True
    for i in range(26):
        if not find[i]:
            print(chr(i+97))
            break
    else:
        print("None")
main()

Solution (Python):
s = input()

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

Solution (Python):
S = 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"]
ls2=["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)):
  for j in range(len(ls)):
    if S[i] == ls[j]:
      if ls[j] in ls2:
        ls2.remove(ls[j])
      break
if len(ls2) == 0:
  print("None")
else:
  print(ls2[0])

Solution (Python):
S=input()

atoz=[chr(i) for i in range(97, 97+26)]

try:
    print(sorted(list(set(atoz)-set(S)))[0])
except:
    print(None)

Solution (Python):
a = set(input())
b = set([chr((ord("a") + i)) for i in range(26)])
print(sorted(list(b-a))[0] if b-a else "None" )

Solution (Python):
word = input()
al = [chr(ord("a") + i) for i in range(26)]
d = "None"
for i in al:
    if not i in word:
        d = i
        break
print(d)

Solution (Python):
word = input()
al = "qwertyuiopasdfghjklzxcvbnm"
d = []
for i in range(len(al)):
    if not al[i] in word:
        d.append(al[i])
if d == []:
    print("None")
else:
    print(sorted(d)[0])

Solution (Python):

S = input()

ans = "None"

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

for i in a:
    if i not in S:
        ans = i
        break

print(ans)

Solution (Python):
s = input()
alfa = [chr(i) for i in range(97, 97+26)]
for i in range(len(alfa)):
    for j in range(len(s)):
        if alfa[i] == s[j]:
            alfa[i] = "-"
for i in range(len(alfa)):
    if alfa[i] != "-":
        ans = alfa[i]
        break
    else:
        ans = "None"
print(ans)

Solution (Python):

S = input()
ab = [chr(ord('a') + i) for i in range(26)]
c = ""

for a in ab:
    if a in S:
        pass
    else:
        c = a
        break

if c == "":
    c = "None"

print(c)

Solution (Python):
s = input()
s = sorted(list(s))
alphabets = set(list('abcdefghijklmopqrstuvwxyz'))
Answer = alphabets - set(s)
Answer = sorted(Answer)
if len(Answer) ==0:
  print('None')
else:
  print(Answer[0])

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

s_set = set(list(s))
s_list = list(s_set)
if len(al) == len(s_list):
  print('None')

for a in al:
  if a not in s_list:
    print(a)
    exit(0)

Solution (Python):
S = input()
ans = "None"
alphabet = [chr(ord("a") + i) for i in range(26)]

for s in alphabet:
    if s not in S:
        ans = s
        break

print(ans)

Solution (Python):
import string

S = str(input())
ans = "None"

for s in string.ascii_lowercase:
    if s not in S:
        ans = s
        break

print(ans)

Solution (Python):
import string

S = input()
ans = "None"

for s in string.ascii_lowercase:
    if s not in S:
        ans = s
        break

print(ans)

Solution (Python):
s = input()
word = ['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 = sorted(s)
if len(set(s)) == len(word):
  print('None')
  exit()
ans = [i for i in word if i not in s]
print(ans[0])

Solution (Python):
S = input()
alpha = [chr(i) for i in range(97,123)]

for s in S:
    if s not in alpha:continue
    alpha.remove(s)
print(alpha[0]) if alpha else print("None")

Solution (Python):
alfa = [chr(i) for i in range(97, 97+26)]
S = str(input())
check = 0
for i in alfa:
    if i not in S:
        check = 1
        print(i)
        break

if check == 0:
    print('None')

Solution (Python):
def main():
    s = str(input())

    if not (1 <= len(s) and len(s) <= 10**5):
        return
    if s.islower() == False:
        return
        
    alph = 'abcdefghijklmnopqrstuvwxyz'

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

if __name__ == "__main__":
    main()

Solution (Python):
import sys

S = input()
alphabet_code = list(range(26))

i = 0
j = 0
while i < len(S):
    while j < 26:
        if ord(S[i]) == alphabet_code[j] + 97:
            alphabet_code[j] = -1
        j += 1
    j = 0
    i += 1

while j < 26:
    if alphabet_code[j] != -1:
        print(chr(alphabet_code[j] + 97))
        sys.exit(0)
    j += 1
print("None")

Solution (Python):


import string
import numpy as np

a=set(list(string.ascii_lowercase)) 
S=set(list(input())) 
ans=sorted(list(a-S)) 

if len(ans)>0: 
    print(ans[0])
else: 
    print("None")

Solution (Python):
print(min(set(map(chr,range(97,123)))-set(input())or["None"]))

Solution (Python):
S = [j for j in input()]
x = [chr(i) for i in range(97,97+26)]
x.append("None")
S.sort()

for p in x:
  if p not in S:break

print(p)

Solution (Python):
S=input()
moji=sorted(list(S))
flg=True
for i in range(97,123):
    if chr(i) not in moji:
        print(chr(i))
        flg=False
        break
if flg: print("None")

Solution (Python):
from sys import stdin
import bisect

s = stdin.readline().rstrip()

li = "abcdefghijklmnopqrstuvwxyz"

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

print("None")

Solution (Python):
s = input()


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



t = 'abcdefghijklmnopqrstuvwxyz'
t = list(t)


if s == t:
	print('None')
else:
	for i in range(len(t)):
		
		if i == 25:
			print(t[i])
			break
		elif s[i]!=t[i]:
			print(t[i])
			break

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

Z=list('abcdefghijklmnopqrstuvwxyz')
for i in Z:
    if i in S:
        continue
    else:
        print(i)
        break
else:
    print('None')

Solution (Python):
S = input()
string = [chr(i) for i in range(97, 97+26)]
ans = True
for i in string:
    if((i in S) == False):
        print(i)
        ans = False
        break
if(ans):
    print("None")

Solution (Python):
s = input()
LetterInS = set(s)
import string
LetterInAlphabets = set(string.ascii_lowercase)
if LetterInAlphabets - LetterInS == set() :
    print("None")
else :
    LetterSurplus = LetterInAlphabets - LetterInS
    ListSurplus = sorted(list(LetterSurplus))
    print(ListSurplus[0])

Solution (Python):
x=input()
s=["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 x.count(i)==0:
    print(i)
    exit()
print("None")

Solution (Python):
s=input()
a="abcdefghijklmnopqrstuvwxyz"
for i in a:
    if not i in s:
        print(i)
        exit()
print('None')

Solution (Python):
import sys
import math

def main():
  S = list(input())
  alphabets = set(list('abcdefghijklmnopqrstuvwxyz'))
  for s in S:
    alphabets.discard(s)
  if len(alphabets) == 0:
    print('None')
  else:
    left = list(alphabets)
    left.sort()
    print(left[0])

main()

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

Solution (Python):
s = input()
for i in "abcdefghijklmnopqrstuvwxyz":
  if i not in s:
    print(i)
    exit()
print("None")

Solution (Python):
s=input()
l=[chr(i) for i in range(ord('a'),ord('z')+1)]
s=set(s)
ans=''
for i in l:
    if i not in s:
        ans=i
        break
    if len(s)==26:
        ans='None'
print(ans)

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

Solution (Python):
string = input()
count_array = [0]*26
alphabets = "abcdefghijklmnopqrstuvwxyz"
cha_to_idx = {cha: i for i, cha in enumerate(alphabets)}
for cha in string:
    count_array[cha_to_idx[cha]] = 1

for i, count in enumerate(count_array):
    if count == 0:
        print(alphabets[i])
        break
    elif (count>=1) & (i == 25):
        print("None")

Solution (Python):
s = input()
l = [a for a in "abcdefghijklmnopqrstuvwxyz" if a not in s]
print(l[0] if l else "None")

Solution (Python):
S = input()
alphabet = 'abcdefghijklmnopqrstuvwxyz';
ans = list(set(alphabet) - set(S))
ans.sort()
if not ans:
   print('None');
else:
    print(ans[0])

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

ans = 'None'
for i in alp:
    if i in s:
        pass
    else:
        ans = i
        break

print(ans)

Solution (Python):
s = input()
abc = "abcdefghijklmgopqrstuvwxyz"
for i in abc:
    if not s.count(i):
        print(i)
        break
else:
    print("None")

Solution (Python):
S = str(input())
a=['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 a:
    a.remove(S[i])
print(None if a==[] else a[0])

Solution (Python):
def examB():
    St = S()
    AZ = [chr(i) for i in range(97, 97 + 26)]
    ans = "None"
    for s in AZ:
        if s not in St:
            ans = s
            break
    print(ans)

import sys
import copy
import bisect
import heapq
from collections import Counter,defaultdict,deque
def I(): return int(sys.stdin.readline())
def LI(): return list(map(int,sys.stdin.readline().split()))
def LS(): return sys.stdin.readline().split()
def S(): return sys.stdin.readline().strip()
mod = 10**9 + 7
inf = float('inf')

examB()

Solution (Python):

s = input()
alphabets = 'abcdefghijklmnopqrstuvwxyz'

for c in alphabets:
    if c not in s:
        print(c)
        break
else:
    print('None')

Solution (Python):
S = sorted(list(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 range(len(l)):
  if not l[i] in S:
    print(l[i])
    break
  if i == len(l) - 1:
    print('None')

Solution (Python):
S = input()
abc = "abcdefghijklmnopqrstuvwxyz"
for a in abc:
    if not S.count(a):
        print(a)
        break
else:
    print("None")

Solution (Python):
s=list(input())
s=set(s)
al=[chr(i) for i in range(97, 97+26)]
for i in range(len(al)):
    if al[i] not in s:
        print(al[i])
        break
    if i==len(al)-1:
        print('None')

Solution (Python):
s=sorted(set(input()))
l=list(range(ord("a"),ord("z")+1))
if len(s) == len(l):
    print("None")
else:
    for i in range(len(s)):
        if ord(s[i]) == l[i]:
            pass
        else:
            print(chr(l[i]))
            exit()
    print(chr(ord(s[-1]) + 1))

Solution (Python):
A = input()
alpha = set('abcdefghijklmnopqrstuvwsyz')
a = set(A)
a = alpha.difference(a)
if len(a) == 0:
  print('None')
else:
  a = sorted(list(a))
  print(a[0])

Solution (Python):
s=list(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 l:
    if i not in s:
        print(i)
        exit()
print("None")