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()
for ch in 'abcdefghijklmnopqrstuvwxyz':
  if ch not in S:
    print(ch)
    exit()
print('None')

Solution (Python):
s = list(input())
alp = list("abcdefghijklmnopqrstuvwxyz")
for i in s:
    for j in range(len(alp)):
        if i == alp[j]:
            alp.pop(j)
            break
if len(alp) != 0: print(alp[0])
else: print("None")

Solution (Python):
inputStr = input()
inputNum = len(inputStr)
string = "abcdefghijklmnopqrstuvwxyz"
count1, count2 = 0, 0
for i in string:
  count2 = 0
  for j in inputStr:
    if i == j:
      break
    else:
      count2 += 1
  if count2 == inputNum:
    break
  count1 += 1
if count1 == 26:
  print("None")
else:
  print(i)

Solution (Python):
S = sorted(set(input()))
alp = 'abcdefghijklmnopqrstuvwxyz'
if len(S) == len(alp):
    print(None)
    exit()
for i in range(len(S)):
    if S[i] != alp[i]:
        print(alp[i])
        exit()
print(alp[len(S)])

Solution (Python):
S=list(input())
A=list(set(S))
A.sort()
alphabet=list("abcdefghijklmnopqrstuvwxyz")
k='None'
for i in alphabet:
    if not i in A:
        k=i
        break
print(k)

Solution (Python):
import string

se = set(list(input()))
alphabet = [s for s in string.ascii_lowercase]

for al in alphabet:
    if al not in se:
        print(al)
        exit()

print('None')

Solution (Python):
s=list(input())
a='abcdefghijklmnopqrstuvwxyz'
a_li=list(a)

s_set=set(s)
a_set=set(a_li)

if s_set==a_set:
    print('None')
else:
    result=list(a_set-s_set)
    result.sort()
    print(result[0])

Solution (Python):
S = list(set(input()))
for i in range(26):
    if chr(97+i) not in S:
        print(chr(97+i))
        break
else:
    print("None")

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

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

alphabet = 'abcdefghijklmnopqrstuvwxyz'
alphabet = list(alphabet)

for i in alphabet:
    if i in s:
        continue
    else:
        print(i)
        exit()

print('None')

Solution (Python):
S = set(input())
Alp = list("abcdefghijklmnopqrstuvwxyz")
LL = [i for i in Alp if i not in S]
if LL  == []:
    print("None")
else:
    print(min(LL))

Solution (Python):
S = set(input())
Alp = list("abcdefghijklmnopqrstuvwxyz")
MM = []
for i in Alp:
    if i not in S:
        MM.append(i)
if MM == []:
    print("None")
else:
    print(min(MM))

Solution (Python):
from collections import Counter,defaultdict,deque
from heapq import heappop,heappush,heapify
import sys,bisect,math,itertools
sys.setrecursionlimit(10**8)
mod = 10**9+7
def inp(): return int(sys.stdin.readline())
def inpl(): return list(map(int, sys.stdin.readline().split()))
def inpln(n): return list(int(sys.stdin.readline()) for i in range(n))

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

Solution (Python):
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']
s=input()
ans=0
for i in list:
	if i not in s:
		print(i)
		ans=i
		break
if ans==0:
	print("None")

Solution (Python):

S = list(input())

setS = set(S)
all_alphabet = set([chr(i) for i in range(97, 97+26)])

not_include_alphabet = list(all_alphabet - setS)
not_include_alphabet.sort(reverse=False)

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

Solution (Python):
S = input()
alphabet = "abcdefghijklmnopqrstuvwxyz"
flag = False

for char in alphabet:
    if char not in sorted(set(S)):
        print(char)
        flag = True
        break

if not flag:
    print("None")

Solution (Python):
s = input()
ascii_lowercase = 'abcdefghijklmnopqrstuvwxyz'
for c in s:
    ascii_lowercase = ascii_lowercase.replace(c, "")
if(len(ascii_lowercase) == 0):
    print('None')
else:
    print(ascii_lowercase[0])

Solution (Python):
from sys import stdin
from string import ascii_lowercase
s = stdin.readline().rstrip()
not_used = sorted(set(ascii_lowercase) - (set(s) & set(ascii_lowercase)))
print(not_used[0] if not_used else 'None')

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

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

Solution (Python):
s = 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' ]

b = [0] * 26

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

jug = True

for i in range(26):
    if b[i] == 0:
        print(a[i])
        jug = False
        break

if jug == True:
    print("None")

Solution (Python):
S=list(input())
S.sort()
c=[]
m=["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(m)):
  if S.count(m[i])==0:
    print(m[i])
    break
  else:
    c.append(m[i])
if c==m:
  print("None")

Solution (Python):
S = input()
tmp = list("abcdefghijklmnopqrstuvwxyz")
ans = []
for i in range(len(tmp)):
    if tmp[i] not in S:
        ans.append(tmp[i])
if len(ans) == 0:
    print("None")
else:
    print(ans[0])

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

for i in abc:
    if i not in a:
        print(i)
        break
    
    if i == 'z':
        print("None")

Solution (Python):
alphabet="abcdefghijklmnopqrstuvwxyz"
import collections
table=collections.defaultdict(int)
for x in input():
  table[x]+=1
for x in alphabet:
  if x in table:
    continue
  else:
    print(x)
    exit(0)
print("None")

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

s=input()
for i in range(26):
  temp=alp[i]
  if temp not in s:
    print(temp)
    exit()
print("None")

Solution (Python):
import sys
from functools import reduce
import copy
import math
from pprint import pprint
import collections
import bisect


sys.setrecursionlimit(4100000)


def inputs(num_of_input):
    ins = [input() for i in range(num_of_input)]
    return ins


def int_inputs(num_of_input):
    ins = [int(input()) for i in range(num_of_input)]
    return ins


def solve(inputs):
    s = inputs[0]
    c_set = set()

    for c in s:
        c_set.add(c)

    for c in 'abcdefghijklmnopqrstuvwxyz':
        if c not in c_set:
            return c
    return 'None'


def string_to_int(string):
    return list(map(int, string.split()))


if __name__ == "__main__":
    ret = solve(inputs(1))
    print(ret)

Solution (Python):
s = input()

s_s = set(s)
s_a = set(map(chr, range(ord('a'), ord('z') + 1)))
r = ''.join(s_a - s_s) 

if not r:
    print('None')
else:
    print(sorted(r)[0])

Solution (Python):
Labc=[chr(i) for i in range(97,97+26)]
Sabc=set(Labc)

S=input()

SS=set()
for c in S:
    SS.add(c)

L=sorted(list(Sabc-SS))
print(L[0] if len(L)>0 else 'None')

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

alphabet = [chr(ord("a") + i) for i in range(26)]
ans = "None"
for actual, expected in zip(s, alphabet):
    if actual != expected:
        ans = expected
        break

if ans == "None" and s[len(s)-1] != "z":
    ans = alphabet[len(s)]
print(ans)

Solution (Python):
S = set(input())
A = set(map(chr, range(ord('a'), ord('z')+1)))

R = A - S

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

Solution (Python):
d='abcdefghijklmnopqrstuvwxyz'
s=input()
for i in d:
  if i not in s:
    print(i)
    break
else:
  print('None')

Solution (Python):
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']
s=input()
ans=0
for i in list:
	if i not in s:
		print(i)
		ans=i
		break
if ans==0:
	print("None")

Solution (Python):
n = sorted(list(map(str, input())))
alpha = [chr(ord('a') + i) for i in range(26)]
ans = 'None'

for i in range(26):
  if alpha[i] not in n:
    ans = alpha[i]
    break
print(ans)

Solution (Python):
S=set(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']

if len(S)==26:
    print('None')
else:
    for i in range(26):
        if l[i] not in S:
            print(l[i])
            break

Solution (Python):
print(min(set(map(chr,range(ord("a"),ord("z")+1)))-set(input())or["None"]))

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

Solution (Python):
s = input()

s_set = set(s)
s_list = list(s_set)
s_list.sort()

afb = "abcdefghijklmnopqrstuvwxyz"

flag = True
for i in afb:
    if i not in s_list:
        flag = False
        print(i)
        break
if flag:
    print("None")

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

Solution (Python):
s=input()
tmp='a'
ans=''
while True:
    if s.count(tmp)==0:
        ans=tmp
        break
    if tmp=='z':
        ans=None
        break
    tmp=chr(ord(tmp)+1)
print(ans)

Solution (Python):


S = input()

set_S=set(S)

alpha = "abcdefghijklmnopqrstuvwxyz"

productSet = sorted(set_S^set(alpha))

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

Solution (Python):
s = input()
alphabet = list("abcdefghijklmnopqrstuvwxyz")
for char in alphabet:
  if char not in s:
    print(char)
    exit()
else:
  print("None")

Solution (Python):
import string

S = list(input())
T = list(string.ascii_lowercase)
found = 0
for i in range(0, 26):
  for j in range(0, len(S)):
    if T[i] == S[j]:
      break
    if j == len(S)-1:
      print(T[i])
      found = 1
  if found == 1:
    break
  if i == 25:
    print('None')

Solution (Python):
S = list(input())
alphabets = [chr(i) for i in range(97, 97 + 26)]
finds = []
for a in alphabets:
    if a not in S and a not in finds:
        finds.append(a)
if len(finds) == 0:
    print('None')
else:
    print(finds[0])

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

s = input()

flag = True

for c in arr:
    if c not in s:
        print(c)
        flag = False
        break
        
if flag:
    print("None")

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

Solution (Python):
s = sorted(set(input()))
r = "abcdefghijklmnopqrstuvwxyz"
if len(r)-len(s)!=0:
  s +=[""]*(len(r)-len(s))
for i, j in zip(s, r):
  if i != j:
    print(j)
    quit()
print("None")

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

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

Solution (Python):
s=input()
b=len(s)
a="abcdefghijklmnopqrstuvwxyz"
i=0
while i<b and a!=[]:
  a=a.replace(s[i],"")
  i=i+1
if len(a)!=0:
  print(a[0])
else:
  print("None")

Solution (Python):
s=list(set((str(input()))))
s.sort()
s="".join(map(str,s))
t='abcdefghijklmnopqrstuvwxyz'
if s=='abcdefghijklmnopqrstuvwxy':
    print('z')
else:
    for i in range(len(s)):
        if s[i]!=t[i]:
            print(t[i])
            break
    else:
        print(None)

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

Solution (Python):
import sys
S = input()
alphabet = "abcdefghijklmnopqrstuvwxyz"

for s in alphabet:
    if s not in S:
        print(s)
        sys.exit()

print("None")

Solution (Python):
S = input()

alphabet = 'abcdefghijklmnopqrstuvwxyz'

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

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

Solution (Python):
s = [c  for c in input()]
for i in 'abcdefghijklmnopqrstuvwxyz':
  if s.count(i)==0:
    print(i)
    exit()
print('None')

Solution (Python):
s = sorted(set(input()))
for i in range(len(s)):
    if ord(s[i]) != ord("a")+i:
        ans = chr(ord(s[i])-1)
        break
    ans = chr(ord(s[-1]) + 1)
if len(s) == 26:
    ans = "None"
print(ans)

Solution (Python):
S = sorted(set(list(input())))
if len(S)==26:
    print("None")
else:
    for i in range(97,123):
        if chr(i) not in S:
            print(chr(i))
            break

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

Solution (Python):
S=set(input())
Al="qwertyuiopasdfghjklzxcvbnm"
AL=list(Al)
l=[]
for i in range(len(Al)):
  if not AL[i] in S:
    l.append(AL[i])
if l!=[]:
  print(sorted(l)[0])
else:
  print("None")

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

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

Solution (Python):

S = input()

ABC = 'abcdefghijklmnopqrstuvwxyz'

for s in S:
    if s in ABC:
        ABC = ABC.replace(s,'')

if len(ABC) == 0:
    ans = 'None'
else:
    ans = ABC[0:1]

print(ans)

Solution (Python):
D="abcdefghijklmnopqrstuvwxyz"
S=D.strip(input())
if S:print(S[0])
else:print("None")

Solution (Python):
S=list(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','None']

S.sort()

for s in alphabet:
  if s in S:
    pass
  else:
    print(s)
    break

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

for i in range(26):
  if chr(i+97) not in S:
    print(chr(97+i))
    break
    
else:
  print('None')

Solution (Python):
s = input()

alpha = 'abcdefghijklmnopqlstuvwxyz'

flag = True
for _ in alpha:
    if _ not in s:
        flag = False
        print(_)
        break

if flag:
    print('None')

Solution (Python):
s = set(input())
target = set(list("abcdefghijklmnopqrstuvwxyz"))  
non = list(target-s)
non.sort()
print(non[0] if len(non) != 0 else "None")

Solution (Python):
def slove():
    import sys
    import collections
    input = sys.stdin.readline
    ls = list("abcdefghijklmnopqrstuvwxyz")
    d = collections.defaultdict(int)
    for v in ls:
        d[v]
    s = str(input().rstrip('\n'))
    for i in range(len(s)):
        d[s[i]] = 1
    for k, v in d.items():
        if v == 0:
            print(k)
            exit()
    print("None")


if __name__ == '__main__':
    slove()

Solution (Python):
s = set(input())
alpha = set(map(chr, range(97, 123)))

print(sorted(s^alpha)[0] if len(s)!=26 else 'None')

Solution (Python):
S=input()
s="abcdefghijklnmopqrstuvwxyz"
s = list(s)
  
for si in S:
  if si in s:
  	s.remove(si)
    
if s == []:
  print("None")
else:
  print(s[0])

Solution (Python):
S = input()
s = "abcdefghijklnmopqrstuvwxyz"
s = list(s)

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

if s == []:
    print("None")

else:
    print(s[0])

Solution (Python):
S=input()
def f(S):
    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 l in L:
        if l not in S:
            return print(l)
    return print("None")
f(S)

Solution (Python):
s=input()

x='abcdefghijklmnopqrstuvwxyz'
          
for i in x:
    if i not in s:
        print(i)
        exit()

print('None')

Solution (Python):
chars="abcdefghijklmnopqrstuvwxyz"
s=input()
res=None
for i in range(len(chars)):
  if chars[i] not in s:
    res=chars[i]
    break

print(res)

Solution (Python):
s = list(input())
s.sort()
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']
no_include = []
sum = 0

for i in alpha:
    if i not in s:
        no_include.append(i)
        sum += 1

if sum != 0:
    print(no_include[0])
else:
    print('None')

Solution (Python):
s=input()
for i in range(97,97+26):
    if chr(i) not in s:
        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 s in S:
  if s in alpha:
    alpha.remove(s)
if len(alpha) == 0:
  print('None')
else:
  print(alpha[0])

Solution (Python):
import string

s=list(set(input()))

for i in list(string.ascii_lowercase):
    if i not in s:
        print(i)
        exit()
print('None')

Solution (Python):
S=input()
d={}
for i in range(ord('a'),ord('z')+1):
	d[i]=0
for s in S:
	d[ord(s)]+=1

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

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

print(target)

Solution (Python):
S=input()
count=[0]*26
c=ord('a')
for s in S:
    count[ord(s)-c]+=1
for i,k in enumerate(count):
    if k == 0:
        print(chr(i+c))
        exit()
print('None')

Solution (Python):
a = 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']
if len(set(a))==26:
  print('None')
else:
  for i in range(len(l)):
    if l[i] not in a:
      print(l[i])
      break

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

a = 'abcdefghijklmnopqrstuvsxyz'

for c in a:
    if c not in s:
        print(c)
        exit(0)

print('None')

Solution (Python):
a = set("qwertyuiopasdfghjklzxcvbnm")
s = set(input())
b = a-s
print(sorted(list(b))[0] if len(b) > 0 else "None")

Solution (Python):
import sys
S = input()
s_el = set(S)
for a in range(97, 97+26):
    if chr(a) not in s_el:
        print(chr(a))
        sys.exit()
print('None')

Solution (Python):
s=input()
alp='abcdefghijklmnopqrstuvwxyz'
for i in range(26):
  alp=[j for j in alp if j not in s]
if len(alp)!=0:
  print(alp[0])
else:
  print('None')

Solution (Python):
S = list(input())
T = []

for i in S:
        tmp = ord(i)
        T.append(tmp)

for j in range(ord("a"),ord("z")+1):
        if j not in T:
                print(chr(j))
                exit()

print("None")

Solution (Python):
import string
s = input()
l = string.ascii_lowercase
for i in range(len(l)):
    if not (l[i] in s):
        print(l[i])
        exit()
print("None")

Solution (Python):
import string
a = set(string.ascii_lowercase)
S = input()
if sorted(list(set(S))) == list(string.ascii_lowercase):
    print("None")
else:
    for i in set(S):
        if i in a:
            a.remove(i)
    print(sorted(list(a))[0])

Solution (Python):
s=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']
if len(set(s))==26:
    print('None')
else:
    for i in range(26):
        if l[i] not in s:
            print(l[i])
            break

Solution (Python):
s = set(list(input()))
l = set(list('qawsedrfgtyhujikolpzxcvbnm'))
a = list(l-s)
a.sort()
if len(a)==0:
  print('None')
else:
  print(a[0])

Solution (Python):
S = set(list(input()))
s = list(S)
c = 'abcdefghijklmnopqrstuvwxyz'

if len(s) == 26:
    print('None')
else:
    for i in c:
        if i not in s:
            print(i)
            exit()

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

if len(s) == 26:
    print('None')
else:
    for i in c:
        if i not in s:
            print(i)
            exit()

Solution (Python):



S = input()


flag = False
for i in range(97, 97 + 26):
    if chr(i) not in S:
        ans = chr(i)
        flag = True
        break



print(ans if flag else 'None')

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

Solution (Python):
s = set(list(input()))
t = set(list('qwertyuiopasdfghjklzxcvbnm'))
u = sorted(t-s)
if len(u) == 0:
    print('None')
else:
    print(u[0])

Solution (Python):
import string
S = input()
D = {}
for s in S:
  D[s] = 1

for c in string.ascii_lowercase:
  if c not in D:
    print(c)
    exit()
print("None")

Solution (Python):
S = input()
set_S = set(list(S))
smallest = ""
for c in list("abcdefghijklmnopqrstuvwxyz"):
    if c not in set_S:
        smallest = c
        break
        
if smallest == "":
    print("None")
else:
    print(smallest)

Solution (Python):
import string
s=input()
alp=string.ascii_lowercase
for a in alp:
  if not(a in s):
    print(a)
    exit()
print("None")

Solution (Python):
import string
alpha = list(string.ascii_lowercase)
s = input()
li = [st for st in alpha if st not in s]
if len(li) == 0:
    print('None')
elif len(li) > 0:
    print(li[0])

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


def main():
    S = set(list(input()[:-1]))
    for s in string.ascii_lowercase:
        if s not in S:
            print(s)
            return
    print('None')


if __name__ == '__main__':
    main()

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

Solution (Python):
s = input()

arr = list(s)

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

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

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

Solution (Python):
s = input()
s_sorted = sorted(list(set(s)))
for i in range(97, 97+26):
    if chr(i) not in s_sorted:
        print(chr(i))
        exit()
    else:
        s_sorted.remove(chr(i))
print('None')

Solution (Python):
s=list(input())
alpha=[chr(i) for i in range(97, 97+26)]
check=[]
for c in alpha:
    if c not in s:
        check+=[c]
if(check==[]):
    print("None")
else:
    check.sort()
    print(check[0])

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

Solution (Python):
alphabet = 'abcdefghijklmnopqrstuvwxyz'
s = input()
out = None

for i in range(0,len(alphabet)):
  if alphabet[i] not in s:
    out = alphabet[i]
    break
print(out)

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

ans="None"
for t in list("abcdefghijklmnopqrstuvwxyz"):
  if t not in S:
    ans=t
    break
print(ans)

Solution (Python):
S=list(input())
T=list("abcdefghijklmnopqrstuvwxyz")

ans="None"
for t in T:
  if t in S:
    1
  else:
    ans=t
    break
print(ans)

Solution (Python):
import sys
import math
from fractions import gcd





def lcm(a, b):
    return a * b // gcd(a, b)

def combination_count(n, r):
    return math.factorial(n) // (math.factorial(n - r) * math.factorial(r))

def permutations_count(n, r):
    return math.factorial(n) // math.factorial(n - r)

big_prime = 1000000007

"""


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


 = int(sys.stdin.readline())
 = map(int, sys.stdin.readline().split())
 = list(map(int, sys.stdin.readline().split()))
 = [list(map(int,list(sys.stdin.readline().split()))) for i in range(N)]
"""

alp = "abcdefghijklmnopqrstuvwxyz"
S = sys.stdin.readline().rstrip()

for a in alp:
    if a not in S:
        print(a)
        exit()
print("None")

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

Solution (Python):
import collections
import string

s = input().rstrip()
counter = collections.Counter()
for c in s:
    counter[c] += 1
for c in string.ascii_lowercase:
    if counter[c] == 0:
        print(c)
        break
else:
    print('None')

Solution (Python):
import sys
import string

alphabets = string.ascii_lowercase
s = input()

for i in alphabets:
    if i not in s:
        print(i)
        sys.exit()

print("None")

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

Solution (Python):
import sys

if sys.platform =='ios':
    sys.stdin=open('input_file.txt')


S=input()

Alp="qwertyuiopasdfghjklzxcvbnm"

Q=sorted(set(S)^set(Alp))

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

Solution (Python):
S = input()

S_list = []
for s in S:
    if s not in S_list:
        S_list.append(s)

S_list.sort()

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

check_list = []
for i in range(len(S_list)):
    if S_list[i] == al_list[i]:
        check_list.append(True)
    else:
        check_list.append(False)

count = 0
for c in check_list:
    if c is True:
        count += 1
    else:
        count += 0

if count == 26:
    print('None')
else:
    print(al_list[count])

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

ki=[0]*26

for i in range(len(s)):
    for j in range(26):
        if s[i]==al[j]:
            ki[j]+=1
for i in range(26):
    if ki[i]==0:
        print(al[i])
        exit()
print('None')

Solution (Python):
S = set(list(input()))
a = {chr(ord('a') + i) for i in range(26)}
d = a - S
print('None' if not len(d) else min(d))

Solution (Python):
import string
s = input()
k = string.ascii_letters
alist = [0]*26
for i in s:
    alist[ord(i)-ord("a")] = 1
c = -1
for i in range(26):
    if alist[i] == 0:
        c = i
        break    
if c == -1:
    print("None")
else:
    print(k[c])

Solution (Python):
s = list(set(list(input())))
a = list("abcdefghijklmnopqrstuvwxyz")
for c in s:
  a.remove(c)
print(a[0]) if len(a)>0 else print("None")

Solution (Python):
import string
S = set([c for c in input()])
for c in string.ascii_lowercase:
    if c not in S:
        print(c)
        exit()
print("None")

Solution (Python):
S = input()
d = {}
for s in S:
    d[s] = 1
a = [chr(i) for i in range(97, 97+26)]
ans = "None"
for i in a:
    if not i in S:
        ans = i
        break
print(ans)

Solution (Python):
def resolve():
    s = input()
    t = set([c for c in "abcdefghijklmnopqrstuvwxyz"])
    st = sorted(set(t) - set(s))
    print(st[0] if st else "None")


if __name__ == "__main__":
    resolve()

Solution (Python):
S=input()
cnt=0
for i in range(ord('a'), ord('z')+1):
  if chr(i) not in S:
    print(chr(i))
    exit()
  else:
    cnt+=1
if cnt==(ord('z')+1-ord('a')):
  print('None')

Solution (Python):
S = input()
S_list = list(set(S))
string = "qazwsxedcrfvtgbyhnujmikolp"
str_list = list(string)
str_list.sort()
for a in S_list:
    str_list.remove(a)
if str_list == []:
    print("None")
else:
    print(str_list.pop(0))

Solution (Python):
import string

a_z = set(string.ascii_lowercase)

s = set(input())

subtract_alphabet = list(a_z - s)
if len(subtract_alphabet) == 0:
    print("None")
else:
    print(min(subtract_alphabet))

Solution (Python):
import string
S = input()
eigo = string.ascii_lowercase
eigo_list = list(eigo)

count = 0

for x in eigo_list:
    if not  x in S:
        print(x)
        count += 1
        break
    
if count == 0:
    print("None")

Solution (Python):
from collections import deque
char = deque(['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'])

string = list(input())

for i in range(len(string)):
    if string[i] in char:
        char.remove(string[i])

output = char.popleft() if char else "None"
print(output)

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

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

print (r)

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

for i in A:
    if i not in S:
        print(i)
        break
else:
    print("None")

Solution (Python):
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")

Solution (Python):
tester = "abcdefghijklmnopqrstuvwxyz"
s = input()
ans = "None"
for i in range(len(tester)):
    if s.count(tester[i]) == 0:
        ans = tester[i]
        break
print(ans)

Solution (Python):
S = input()

import string

letters = string.ascii_lowercase

S = set(sorted(S))
for l in letters:
    if l not in S:
        print(l)
        break

else:
    print("None")

Solution (Python):
s = input()
abcList = "abcdefghijklmnopqrstuvwxyz"
swi = "None"
for i in range(len(abcList)):
    if s.count(abcList[i]) == 0:
        swi = abcList[i]
        break
print(swi)

Solution (Python):
import string


s = input()
t = set(s)
for ch in sorted(string.ascii_lowercase):
    if not ch in t:
        print(ch)
        exit(0)

print('None')

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

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

S_set = set(S)

ABC = set(list('abcdefghijklmnopqrstuvwxyz'))

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

Solution (Python):
original = 'abcdefghijklmnopqrstuvwxyz'
s = input()
for i in range(len(s)):
    if s[i] in original:
        original = original.replace(s[i], '')
if len(original) > 0:
    print(original[0])
else:
    print(None)

Solution (Python):
import numpy as np

S = input()

S = np.unique(sorted(S))

ans = 'None'
for ch in 'abcdefghijklmnopqrstuvwxyz':
  if ch in S:
    pass
  else:
    ans = ch
    break
print(ans)

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

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

Solution (Python):
s = input()
alp = "abcdefghijklmnopqrstuvwxyz"
for i in s:
  alp = alp.replace(i, "")

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

Solution (Python):
S=set(list(input()))
A='abcdefghjiklmnopqrstuvwxyz'
for i in A:
  if i not in S:
    print(i)
    break
else:
  print('None')

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

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

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

ans = sorted(list(apl-s))
print(ans[0] if len(apl-s)!=0 else 'None')

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

Solution (Python):
s = input()
alphabet = "abcdefghijklmnopqrstuvwxyz"
for chr in alphabet:
  if chr not in s:
    print(chr)
    break
else:
  print("None")