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()
alphabet = 'abcdefghijklmnopqrstuvwxyz'

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

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

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

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

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

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

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

Solution (Python):
import sys
s = input()

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

    
print(None)

Solution (Python):
from string import ascii_lowercase


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

    return not_in_s[0]


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


if __name__ == '__main__':
    main()

Solution (Python):
s = input()

alphabets = 'abcdefghijklmnopqrstuvwxyz'

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

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

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

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

Solution (Python):
import string

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

ans = ""
arr = []

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Solution (Python):
s = input()

alphabets = 'abcdefghijklmnopqrstuvwxyz'

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

print('None')

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

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

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

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

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

Solution (Python):
import string

S = set(input())

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

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

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

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

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

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

Solution (Python):



from math import gcd
from itertools import combinations,permutations,accumulate, product 

from collections import deque,defaultdict,Counter
import decimal
import re
import math
import bisect
import heapq














import sys
sys.setrecursionlimit(10000000)
mod = 10**9 + 7


INF = float('inf')
from sys import stdin
readline = stdin.readline
def readInts():
  return list(map(int,readline().split()))
def readTuples():
    return tuple(map(int,readline().split()))
def I():
    return int(readline())
s = input()
lis = "abcdefghijklmnopqrstuvwxyz"
for i in range(len(lis)):
    if lis[i] in s:
        pass
    else:
        print(lis[i])
        exit()
print('None')

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

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

Solution (Python):
import sys



def input():
    return sys.stdin.readline().strip()


def input_int():
    return int(input())


def input_int_list():
    return [int(i) for i in input().split()]


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


if __name__ == "__main__":
    main()

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

Solution (Python):
S = input().strip()
C = {chr(i):0 for i in range(97,123)}
for i in range(len(S)):
    C[S[i]] += 1
flag = 0
a = 0
for i in range(97,123):
    if C[chr(i)]==0:
        a = chr(i)
        flag = 1
        break
if flag==1:
    print(a)
else:
    print("None")

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):
s = list(str(input()))
l = set(s)
for i in range(26):
    c = chr(ord('a')+i)
    if c not in l:
        print(c)
        exit()
else:
    print('None')

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

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

Solution (Python):

import string

S = str(input())

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

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

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

Solution (Python):

import sys
input=sys.stdin.readline

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

if __name__=="__main__":
    main()

Solution (Python):
S = list(input())
alpha = 'abcdefghijklmnopqrstuvwxyz'

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

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

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

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

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

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

print("None")

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

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

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

Solution (Python):
L = [chr(i) for i in range(97, 123)]
f = 0
s = input()
for l in L:
  if s.count(l) == 0:
    print(l)
    f = 1
    break
if f == 0:
  print("None")

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

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

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

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

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

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

Solution (Python):
def main():
    s = input()
    l = "abcdefghijiklmopqrstuvwxyz"

    for i in range(len(s)):
        l = l.replace(s[i], '')

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


if __name__ == "__main__":
    main()

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

Solution (Python):
S = input()

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

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

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

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

Solution (Python):
import sys
input = sys.stdin.readline
sys.setrecursionlimit(10 ** 7)

import string

s = set(input().strip())
alpha = set(string.ascii_lowercase)
s = sorted(list(alpha - s))
if s:
    print(s[0])
else:
    print('None')

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

alphabet = list('abcdefghijklmnopqrstuvwxyz')

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

print('None')

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

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

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

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

Solution (Python):
alp = "abcdefghijklmnopqrstuvwxyz"
ans = [False for _ in range(26)]
S = list(input())
for i in S:
    for p in range(26):
        if i == alp[p]:
            ans[p] = True
for i in range(26):
    if ans[i] == False:
        print(alp[i])
        break
else:
    print("None")

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

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

import string

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


if __name__=="__main__":
    main()

Solution (Python):
S = input()
a = "abcdefghijklmnopqrstuvwxyz"
ans = "None"
for x in a:
  if x not in S:
    ans = x
    break
print(ans)

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

Solution (Python):

S = input()
c = [0]*27
for i in range(len(S)):
    c[ord(S[i])-ord('a')] = 1
i = 0
while c[i]:
    i += 1
ans = chr(ord('a')+i) if i<26 else 'None'
print(ans)

Solution (Python):



from fractions import gcd
from itertools import combinations,permutations,accumulate, product 

from collections import deque,defaultdict,Counter
import decimal
import re
import math
import bisect














import sys
sys.setrecursionlimit(10000000)
mod = 10**9 + 7


from sys import stdin
readline = stdin.readline
def readInts():
  return list(map(int,readline().split()))
def readTuples():
    return tuple(map(int,readline().split()))
def I():
  return int(readline())
ans = "abcdefghijklmnopqrstuvwxyz"
s = input()
s = sorted(list(set(s)))
for v in ans:
    flag = True
    for j in range(len(s)):
        if s[j] == v:
            flag = False
    if flag:
        print(v)
        exit()
print(None)

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

Solution (Python):
S = input()

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

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

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

print("None")

Solution (Python):
from collections import Counter

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

alp = 'abcdefghijklmnopqrstuvwxyz'

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

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

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):
S = input().strip()
C = {chr(i):0 for i in range(97,123)}
for i in range(len(S)):
    C[S[i]] += 1
x = "None"
for i in range(97,123):
    if C[chr(i)]==0:
        x = chr(i)
        break
print(x)

Solution (Python):
from string import ascii_lowercase
S = input()
for c in ascii_lowercase:
    if c not in S:
        print(c)
        exit()
print('None')

Solution (Python):
s = input()

for i in range(97,123):
	if not chr(i) in s:
		print(chr(i))
		exit()

print("None")

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

Solution (Python):
import sys
import math
import itertools
import collections
import heapq
import re
import numpy as np
from functools import reduce

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

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

Solution (Python):
S=input()

alph="abcdefghijklmnopqrstuvwxyz"

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

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

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

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

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

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 = str(input())

alp = set("abcdefghijklmnopqrstuvwxyz")
s = set(s)


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

print(ans)

Solution (Python):
import sys

readline = sys.stdin.readline
MOD = 10 ** 9 + 7
INF = float('INF')
sys.setrecursionlimit(10 ** 5)


def main():
    from string import ascii_lowercase

    s = set(input())
    t = set(ascii_lowercase)
    u = t - s

    if u:
        v = sorted(list(u))
        print(v[0])
    else:
        print("None")


if __name__ == '__main__':
    main()

Solution (Python):


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

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

print(ans)

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

Solution (Python):
s=input()

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

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

S.sort()

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

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

Solution (Python):
import string

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

ans = sorted(list(all_string - S))

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

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"]
l = list(set(alpha) - set(s))
l.sort()
print(l[0] if len(l) != 0 else "None")

Solution (Python):
S = input()

a = list('abcdefghijklmnopqrstuvwxyz')

n=[]
for i in a:
  if i not in S:
    n.append(i)
    
if len(n)==0:
  print('None')
  
else:
  print(min(n))

Solution (Python):
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']
s=str(input())
for i in range(len(a)):
  if a[i]not in s:
    print(a[i])
    exit()
print("None")

Solution (Python):



def main():
    s = input()
    li = [0] * 26
    for i in list(s):
        n = ord(i) - ord("a")
        li[n] = 1
    for i in range(26):
        if li[i] == 0:
            print(chr(i + ord("a")))
            return

    print("None")


if __name__ == "__main__":
    main()

Solution (Python):
s= input()

for i in range(ord('a'), ord('z') + 1):
    if s.count(chr(i)) == 0:
        print(chr(i))
        exit()

print('None')

Solution (Python):
S = input()

ans = "None"

for i in range(ord("a"), ord("z") + 1):
    if not chr(i) in S:
        ans = chr(i)
        break
print(ans)

Solution (Python):
S = input()

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

Solution (Python):
S = input()
alp = 'abcdefghijklmnopqrstuvwxyz'
rst = 'None'
for i in alp:
	if not i in S:
		rst = i
		break
print(rst)

Solution (Python):
import string

S = list(input())

low_list = list(string.ascii_lowercase)
for i in range(len(S)):
    try:
        low_list.remove(S[i])
    except ValueError:
        pass

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

Solution (Python):
s=input()
al=['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(26):
  if al[i] not in s:
    print(al[i])
    exit()
print('None')

Solution (Python):
s = input()

a = ord('a')

t = set()
for c in s:
    t.add(ord(c) - a)

for i in range(26):
    if not i in t:
        print(chr(a + i))
        exit()

print("None")

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

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

s.sort()

for i in range(26):
    if i >= len(s) or s[i] != chr(i + ord('a')):
        print(chr(i + ord('a')))
        exit()
else:
    print('None')

Solution (Python):
n = input()
a = list('abcdefghijklmnopqrstuvwxyz')
a = set(a)
v = []
for i in a:
    if not i in n:
        v.append(i)
if len(v) == 0:
    print('None')
else:
    v.sort()
    print(v[0])

Solution (Python):



def main():
    a = set("abcdefghijklmnopqrstuvwxyz") - set(input())
    if a == set():
        print("None")
        exit(0)
    print(sorted(list(a))[0])


main()

Solution (Python):
s=str(input())
X="abcdefghijklmnopqrstuvwxyz"
Y=[]
for i in range(len(X)):
    Y.append(X[i])
for i in range(len(X)):
    if X[i] in s:
        Y.remove(X[i])


if len(Y)==0:
    print("None")

else:
    print(Y[0])

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

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

Solution (Python):
def main():
    s = input()
    s = sorted(s)
    ascii_lowercase = 'abcdefghijklmnopqrstuvwxyz'
    ans = 'None'
    for chr_alpha in ascii_lowercase:
        if chr_alpha not in s:
            ans = chr_alpha
            break
    print(ans)


if __name__ == "__main__":
    main()

Solution (Python):
s=input()
for i in range(26):
    if not chr(97+i) in s:
        print(chr(97+i))
        exit()
print("None")

Solution (Python):
S = input()

s_list = [s for s in S]
c_list = [c for c in "abcdefghijklmnopqrstuwvxyz"]

answer = "None"
for c in c_list:
  if c not in s_list:
    answer = c
    break

print(answer)

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

Solution (Python):
import string
S = list(input())
ar = string.ascii_lowercase
for w in ar:
  if w not in S:
    print(w)
    exit()
print('None')

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

Solution (Python):
s=input()

alp="abcdefghijklmnopqrstuvwxyz"

ans="None"
for i in alp:
  if i not in s:
    ans=i
    break
    
print(ans)

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

Solution (Python):
S=input()

alphabet="abcdefghijklmnopqrstuvwxyz"

ans = "None"

for i in range(len(alphabet)):
    if alphabet[i] in S:
        continue
    else:
        ans = alphabet[i]
        break
print(ans)

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

for c in str:
    found = False
    for s in S:
       if s == c:
           found = True
           break
    if found:
        continue
    else:
        print(c)
        exit()

print('None')

Solution (Python):
s = str(input())
ans = 'None'
for j in range(97, 123):
    if chr(j) in s:
        continue
    else:
        ans = chr(j)
        break
print(ans)

Solution (Python):
S = input()
S = sorted(S)
ans = 'None'
for i in range(97, 123):
    if S.count(chr(i)) > 0:
      continue
    else:
      ans = chr(i)
      break
print(ans)

Solution (Python):
s=input()
l='abcdefghijklmnopqrstuvwxyz'
for i in range(26):
    if(s.count(l[i])==0):
        print(l[i])
        exit()
print('None')

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']
for i in range(len(s)):
  try:
    l.remove(s[i])
  except:
    continue
try:
  print(l[0])
except:
  print("None")

Solution (Python):
S = input()
N = len(S)

X = set()
ans = 'None'

for i in range(N):
    X.add(S[i])

for alph in ['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 alph not in X:
             ans = alph
             break

print(ans)

Solution (Python):
s = input()

for i in range(26):
    x = chr(i + 97)
    if not x in s:
        print(x)
        exit()
print('None')

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

Solution (Python):
a=list(input())
n=[]
for i in range(97, 123):
    n.append(chr(i))
a=list(set(a))
ans=[]
for i in n:
    if i not in a:
        ans.append(i)
if ans==[]:
    print('None')
else:
    print(ans[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']

for x in l:
    if x not in S:
        print(x)
        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']
l = []
for i in alpha:
    if i not in s:
        l.append(i)

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

Solution (Python):
S = input()
l = 'abcdefghijklmnopqrstuvwxyz'
for i in range(len(l)):
    if l[i] not in S:
        print(l[i])
        exit(0)
else:
    print('None')

Solution (Python):
import sys,collections as cl,bisect as bs
sys.setrecursionlimit(100000)
input = sys.stdin.readline
mod = 10**9+7
Max = sys.maxsize
def l(): 
    return list(map(int,input().split()))
def m(): 
    return map(int,input().split())
def onem(): 
    return int(input())
def s(x): 
    a = []
    if len(x) == 0:
        return []
    aa = x[0]
    su = 1
    for i in range(len(x)-1):
        if aa != x[i+1]:
            a.append([aa,su])
            aa = x[i+1]
            su = 1
        else:
            su += 1
    a.append([aa,su])
    return a
def jo(x): 
    return " ".join(map(str,x))
def max2(x): 
    return max(map(max,x))
def In(x,a): 
    k = bs.bisect_left(a,x)
    if k != len(a) and a[k] ==  x:
        return True
    else:
        return False

def pow_k(x, n):
    ans = 1
    while n:
        if n % 2:
            ans *= x
        x *= x
        n >>= 1
    return ans

"""
def nibu(x,n,r):
    ll = 0
    rr = r
    while True:
        mid = (ll+rr)//2

    if rr == mid:
        return ll
    if (ここに評価入れる):
        rr = mid
    else:
        ll = mid+1
"""

ss = list(input()[:-1])

ss.sort()

aaa = s(ss)

ima = set()

for i in aaa:
    ima.add(i[0])

data = "qwertyuiopasdfghjklzxcvbnm"

se = set()
for i in data:
    se.add(i)
ans = se - ima

ans = sorted(list(ans))

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

Solution (Python):
import string

s = set(sorted(input()))

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

Solution (Python):
a = "abcdefghijklmnopqrstuvwxyz"
S = str(input())
l = []

for i in range(26):
    if not a[i] in S:
        l.append(i)

if not l:
    print('None')

else:
    print(a[min(l)])

Solution (Python):
S=list(str(input()))
x="abcdefghijklmnopqrstuvwxyz"
ans="None"
for i in range(len(x)):
    if x[i] not in S:
        ans=x[i]
        break
print(ans)