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):
import string


S = list(input())
F = False

for s in list(string.ascii_lowercase):
    if s in S:
        continue
    else:
        print(s)
        F = True
        break

if F == False:
    print('None')

Solution (Python):

def db(x): 
    global debug
    if debug: print(x)
debug = False
S = input()
alpha = [chr(ord('a')+i) for i in range(26)]
for c in S: 
    if c in alpha: alpha.remove(c)
db(alpha)
if alpha==[]: ans = 'None'
else: ans = alpha[0]
print(ans)

Solution (Python):
S = input()
chrs = [0]*26
for i in S:
    chrs[ord(i)-97]+=1
ans = 'None'
for index,i in enumerate(chrs):
    if(i==0):
        ans=chr(index+97)
        break
print(ans)

Solution (Python):

word = input()
alpha_lists = [-1] * 26
flg = True

for w in word:
  alpha_lists[ord(w) - ord('a')] = 1

for a in range(len(alpha_lists)):
  if(alpha_lists[a] == -1):
    print(chr(ord('a') + a))
    flg = False
    break
    
if(flg):
  print("None")

Solution (Python):
import string

S = input()

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

print("None")

Solution (Python):
s =  sorted(set(input()))
query = 'abcdefghijklmnopqrstuvwxyz'
ans = 'None' 
for i in query:
 if i not in s:
  ans = i
  break 
print(ans)

Solution (Python):
S = sorted(set(input()))
dic = 'abcdefghijklmnopqrstuvwxyz'
for c in S:
    if c in dic:
        dic = dic.replace(c, '')
if len(dic) != 0:
    print(dic[0])
else:
    print('None')

Solution (Python):
s = str(input())
x = 'abcdefghijklmnopqrstuvwxyz'
ans = 'None'
for i in x:
  if i not in s:
    ans = i
    break
print(ans)

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

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

Solution (Python):




def  firstCharNotFound(givenString):
	
	checkList = [False for _ in range(0,26,+1)]
	stringLength = range(0,len(givenString),+1)
	for pos in stringLength:
		charIntVersion = ord(givenString[pos])
		checkList[charIntVersion-97]=True
	charRange = range(0,26,+1)
	firstPosition = -1
	for pos in charRange:
		if checkList[pos]==False:
			firstPosition = pos
			break
	if firstPosition == -1:
		return "None"
	else :
		return chr(firstPosition+97)

if __name__ == '__main__':
	S = input().strip()
	solution=firstCharNotFound(S)
	print("{}".format(solution))

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

Solution (Python):
s=list(input())
s.sort()
l=[chr(ord("a")+i) for i in range(26)]
n=[]
for j in l:
  if j not in s:
    n.append(j)
n.sort()
if len(n)==0:
  print("None")
else:
  print(n[0])

Solution (Python):
s=input()
s1=list(s)
s2=set(s1)
az="abcdefghijklmnopqrstuvwxyz"
az1=list(az)
az2=set(az1)
ans=az2-s2
ans1=list(ans)
ans1.sort()
if len(ans1)!=0:
    print(ans1[0])
else:
    print("None")

Solution (Python):
s = input()
ans = 'a'
for i in range(26):
    if ans not in s:
        print(ans)
        exit()
    ans = chr(ord(ans) + 1)
print("None")

Solution (Python):
import sys


def input():
    return sys.stdin.readline()[:-1]


def ii(t: type = int):
    return t(input())


def il(t: type = int):
    return list(map(t, input().split()))


def iml(N: int, t: type = int):
    return [t(input()) for _ in range(N)]


S = sorted(set(ii(str)))
for i in range(97, 97 + 26):
    if chr(i) not in S:
        print(chr(i))
        exit()
print("None")

Solution (Python):
S=list(input())
ans='A'
for i in range(26):
    if chr(ord('z')-i) not in S:
        ans=chr(ord('z')-i)
if ans=='A':
    print('None')
else:
    print(ans)

Solution (Python):
S = input()
dic = [chr(i) for i in range(ord('a'), ord('z')+1)]
flag = True
for j in range(len(dic)):
    if dic[j] not in S:
        print(dic[j])
        flag = False
        break
if flag:
    print('None')

Solution (Python):
import sys

input_ = [ord(_) for _ in sys.stdin.readlines()[0].strip()]
alphabets = [i + 96 for i in range(1, 27)]
for c in input_:
  if c in alphabets:
    alphabets.remove(c)

res = sorted(alphabets)
if len(res) > 0:
  print(chr(res[0]))
else:
  print('None')

Solution (Python):
s = list(input())
count=0
for i in range(97, 123):
    if(chr(i) not in s):
        print(chr(i))
        count+=1
        break
if(count==0):
    print("None")

Solution (Python):
s = str(input())
s = sorted(s)
d = [0] * 150
for i in range(len(s)):
    d[ord(s[i])] += 1
for i in range(97,122+1):
    if d[i] == 0:
        print(chr(i))
        break
    if i == 122:
        print("None")

Solution (Python):
def abc071_b():
    s = input()
    alpha = [chr(i) for i in range(ord("a"), ord("z") + 1)]
    for char in set(s):
        alpha.remove(char)

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


abc071_b()

Solution (Python):
s=input()
tmp="abcdefghijklmnopqrstuvwxyz"
if len(set(s))==26:
    print("None")
    exit()
for i in tmp:
    if not i in s:
        print(i)
        break

Solution (Python):
import string
pp = list(input())
pp=list(set(pp))
pp.sort()
pp.append("1")
s = ""
x = list(string.ascii_lowercase)
x.append("None")
for j in range(len(pp)):
    if pp[j] != x[j]: 
        s = x[j]
        break
    elif pp[j] =="1":
        s = "None"
print(s)

Solution (Python):
S = sorted(set(input()))
dic = "abcdefghijklmnopqrstuvwxyz"
for s in S:
  if(s in dic):
    dic = dic.replace(s,'')
if(dic != ''):
  print(dic[0])
else:
  print("None")

Solution (Python):
S = sorted(input())
dic = "abcdefghijklmnopqrstuvwxyz"
for s in S:
  if(s in dic):
    dic = dic.replace(s,'')
if(dic != ''):
  print(dic[0])
else:
  print("None")

Solution (Python):
S = input()

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

ans = ""
for temp in range(26):
    temp_s = chr(ord("a") + temp)
    if not temp_s in s_set:
        ans = temp_s
        break
else:
    ans = "None"

print(ans)

Solution (Python):
s = input()
res = "None"
for i in "abcdefghijklmnopqrstuvwxyz":
    if s.count(i) <= 0:
        res = i
        break
print(res)

Solution (Python):
S = input()
ans = 'None'
for a in 'abcdefghijklmnopqrstuvwxyz':
    if S.find(a) < 0:
        ans = a
        break
print(ans)

Solution (Python):



def main():
    """Function."""
    s = str(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"]
    base = set()
    for dum in alpha:
        base.add(dum)

    check = set()
    for i in range(len(s)):
        check.add(s[i])

    dum = base - check

    if dum == set():
        print("None")
    else:
        print(min(list(dum)))


if __name__ == "__main__":
    main()

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

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

Solution (Python):
def main():
    chars = set(input())
    for i in range(26):
        c = chr(ord('a') + i)
        if not c in chars:
            print(c)
            return
    print('None')


if __name__ == '__main__':
    main()

Solution (Python):
s = input()
d = {}
for c in "abcdefghijklmnopqrstuvwxyz":
    d[c] = 0

for c in s:
    d[c] += 1

result = None
for k, v in sorted(d.items(), key=lambda x: x[0]):
    if v == 0:
        result = k

        break

print(result if result else "None")

Solution (Python):
s = input()
alpha = [True for _ in range(26)]
for _s in s:
    alpha[ord(_s) - ord("a")] = False
for i in range(len(alpha)):
    if alpha[i]:
        print(chr(i + ord("a")))
        exit()
print("None")

Solution (Python):
S = input()
for c in 'abcdefghijklmnopqrstuvwxyz':
    if c not in S:
        print(c)
        exit()
print('None')

Solution (Python):
import string
s = set(string.ascii_lowercase) - set(input())
print(min(s) if s else 'None')

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

Solution (Python):
S = input()
msg = "None"
for i in range(26) :
    c = chr(ord("a")+i)
    if(S.find(c) == -1) :
        msg = c
        break

print(msg)

Solution (Python):
S = input()
T = 'abcdefghijklmnopqrstuvwxyz'
diff = sorted(list(set(T) - set(S)))
if diff:
    print(diff[0])
else:
    print('None')

Solution (Python):
from collections import Counter

S = input()

cs = Counter(S)

result = "None"

for i in range(26):
    c = chr(ord("a") + i)
    if c not in cs:
        result = c
        break

print(result)

Solution (Python):

S = set(input())
ans = "None"
for s in "abcdefghijklmnopqrstuvwxyz":
    if s not in S:
        ans = s
        break
print(ans)

Solution (Python):

al = "abcdefghijklmnopqrstuvwxyz"
S = set(input())
for s in al:
    if len(al) == len(S):
        print("None")
        break
    elif s not in S:
        print(s)
        break

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

s=set(s)
ch=set(ch)
s=ch-s
s=sorted(s)

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

Solution (Python):
def not_found_minchar(S):
    for o in range(ord('a'), ord('z') + 1):
        if chr(o) not in S:
            return chr(o)
    return "None"

S = raw_input()
print not_found_minchar(S)

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

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

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

s = set(s)
check = set(check)
s = check - s
s = sorted(s)
if(s):
    print(s[0])
else:
    print('None')

Solution (Python):
S = input()
for c in 'abcdefghijklmnopqrstuvwxyz':
    if not c in S:
        print(c)
        exit()
print('None')

Solution (Python):
s = input()
count = 0
al = "qwertyuiopasdfghjklzxcvbnm"
al_list = [i for i in al]
al_list.sort()
for i in al_list:
    if i not in s:
        print(i)
        break
    else:
        count += 1
if count == len(al_list):
    print("None")

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

s.sort()

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

Solution (Python):
s = input()
dup = set(s)
ans = ''

if   len(dup) == 26:
    print('None')
    exit()
else:
    chrs = sorted(dup)
    for i in range(97,123):
        if chr(i) not in chrs:
            ans = chr(i)
            break
print(ans)

Solution (Python):
S = input()
al = [chr(i) for i in range(97, 97+26)]
S = set(S)
al = set(al)
A = al - S
A = list(A)
A.sort()
if len(A) == 0:
    print("None")
else:
    print(A[0])

Solution (Python):
import collections
s = input()
alpha = [chr(i) for i in range(ord('a'), ord('z')+1)]
c = collections.Counter(s)
for al in alpha:
    if al not in c:
        print(al)
        exit()
print("None")

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

for alf in alfs:
    if alf in s:
        continue
    else:
        print(alf)
        exit()
print('None')

Solution (Python):
S=set(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"]
ans=0
for i in Al:
    if i not in S:
        print(i)
        exit()
print('None')

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

Solution (Python):
flag=False
s=input()
for i in "abcdefghijklmnopqrstuvwxyz":
    if i in s:
        continue
    else:
        print(i)
        flag=True
        break
if flag==False:
    print("None")

Solution (Python):
flag=False
s=input()
for i in "abcdefghijklmnopqrstuvwxyz":
    if i in s:
        continue
    else:
        print(i)
        flag=True
        break
if flag==False:
    print("None")

Solution (Python):
s = input()

set_s = set(s)

abc = 'abcdefghijklmnopqrstuvwxyz'

if len(abc) != 26:
	print('err')

set_abc = set(abc)

set_abc -= set_s

ans =[]

for c in set_abc:
	ans.append(c)
	
ans.sort()

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

Solution (Python):
a=list('abcdefghijklmnopqrstuvwxyz')
s=list(input())
for i in s:
    if a.count(i):
        a.remove(i)
if len(a)==0:
    print('None')
else:
    print(a[0])

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

Solution (Python):
import sys
sin={'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=list(input())
snum=len(list(set(s)))
if snum==26:
    print('None')
    sys.exit(0)

kotae=list(sin-set(s))
kotae.sort()
print(kotae[0])

Solution (Python):
a = "abcdefghijklmnopqrstuvwxyz"
b = input()
count = 0
for i in range(26):
    if a[i] not in b:
        print(a[i])
        count += 1
        break
if count == 0:
    print("None")

Solution (Python):
S = input()
N = len(S)
for i in range(26):
    flag = True
    for j in range(N):
        if S[j] == chr(i + ord("a")):
            flag = False

    if flag:
        print(chr(i + ord("a")))
        exit()

print("None")

Solution (Python):
a = list(input())
b = "abcdefghijklmnopqrstuvwxyz"
c = list(b)

for i in range(len(c)):
  if c[i] not in a:
    print(c[i])
    break
  else:
    continue
else:
  print("None")

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

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

print('None')

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

for item in li:
    if s.find(item) == -1:
        print(item)
        break
else:
    print('None')

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

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

Solution (Python):
S = list(input().rstrip())
ts = 'abcdefghijklmnopqrstuvwxyz'

ns = []

for c in S:
    if c not in ns:
        ns.append(c)


if len(ns) == 26:
    print('None')
else:
    ns.sort()
    for tc in ts:
        if tc not in ns:
            print(tc)
            break

Solution (Python):
j = set('abcdefghijklmnopqrstuvwxyz')
s = set(input())
k = sorted(j - s)

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

Solution (Python):
x = set('abcdefghijklmnopqrstuvwxyz')
s = set(input())
y = sorted(x - s)

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

Solution (Python):
s = input()
chk = "abcdefghijklmnopqrstuvwxyz"
ans = ""
for i in range(len(chk)):
    if chk[i] not in s:
        ans = chk[i]
        break
print(ans if ans != "" else "None")

Solution (Python):
alphabet = map(chr, range(ord("a"), ord("z") + 1))
alphabet = set(list(alphabet))
s = input()
s = set(list(s))
try:
    print(min(alphabet - s))
except:
    print("None")

Solution (Python):
S = list(input())
set(S)
A = "abcdefghijklmnopqrstuvwxyz"
list(A)
count = 0
ans = "None"
an = -1
for i in range(len(A)):
    for j in range(len(S)):
        if A[i] == S[j]:
            count = 1
    else:
        if count == 1:
            count = 0
        elif count == 0:
            an = i
            break
if an != -1:
    print(A[an])
else:
    print("None")

Solution (Python):
s = input()

al = "abcdefghijklmnopqrstuvwxyz"
num = len(al)

for i in range(num):
	if al[i] not in s:
		print(al[i])
		break
else:
	print("None")

Solution (Python):
S = input()
l = [True]*26

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

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

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

Solution (Python):
import string
s=set(input())
d=set(string.ascii_lowercase)
print('None' if len(s)==26 else sorted(list(d-s))[0])

Solution (Python):


s=input()
r='None'
for i in range(ord('a'),ord('z')+1):
  if s.count(chr(i))==0:
    r=chr(i)
    break
print(r)

Solution (Python):
import string

S = str(input())
ans = []

for a in string.ascii_lowercase:
    isExist = False
    for i in range(len(S)):
        if S[i] == a:
            isExist = True
    if isExist == False:
        ans.append(a)

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

Solution (Python):
s=list(input())
res="None"
s.sort()
str="abcdefghijklmnopqrstuvwxyz"

for i in range (26):
    if not(str[i] in s):
        res=str[i]
        break
 
print(res)

Solution (Python):
from sys import stdin


def main() -> None:
    s = next_str()
    print(judge(s))


def judge(s: str) -> str:
    for i in range(ord('a'), ord('z') + 1):
        c = chr(i)
        if c not in s:
            return c
    return "None"


def next_str() -> str:
    result = ""
    while True:
        tmp = stdin.read(1)
        if tmp.strip() != "":
            result += tmp
        elif tmp != '\r':
            break
    return result


if __name__ == '__main__':
    main()

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 s.count(al[i]) == 0:
    print(al[i])
    break
  elif i == 25:
    print("None")

Solution (Python):
a = set([chr(i) for i in range(97,97+26)])
b = set(input())
c = a - b
d = sorted(list(c))
if len(d) == 0:
  print("None")
else:
  print(d[0])

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

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

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

Solution (Python):
A=str(input())
P=[]
for i in range(26):
    p=chr(ord("a")+i)
    P.append(p)

ANS=[]
for i in P:
    if i not in A:
        ANS.append(i)
if len(ANS)==0:
    print("None")
else:
    print(ANS[0])

Solution (Python):
s = raw_input()


ch_lst = "abcdefghijklmnopqrstqvwxyz"



match = 0
for j in ch_lst:
  ch = s.find(j)
  if ch != -1:
    continue
  else:
    match = 1
    break

if match == 0:
  print("None")
elif match == 1:
  print(j)

Solution (Python):
S = input()
X = "abcdefghijklmnopqrstuvwxyz"
S_index = sorted(set(S), reverse = True)
S_list = list(S_index)
X_list = list(sorted(X, reverse = True))
answer = None
for i in X_list:
  for n in S_list:
    if i == n:
      break
    elif n == S_list[-1]:
      answer = i
print(answer)

Solution (Python):
s = sorted(set(input()))
a = "abcdefghijklmnopqrdtuvwxyz"
ans = "None"

for _ in a:
  if _ in s:
    pass
  else:
    ans = _
    break
    
print(ans)

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

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

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

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

Solution (Python):
import sys

s = input()

for c in [chr(i) for i in range(97, 97+26)]:
    if not c in s:
        print(c)
        sys.exit()
print("None")

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

if s==a:
    print('None')
else:
    b=sorted(list(a-s))
    print(str(b[0]))

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

Solution (Python):
S=input()
ans='None'
arr=[0 for _ in range(26)]
for x in S:
    arr[ord(x)-ord('a')]=1

for i,x in enumerate(arr):
    if x == 0:
        ans=chr(ord('a')+i)
        break
print(ans)

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

	start = ord('a')
	end = ord('z')

	for i in range(start,end+1):
		
		flg = 0
		for c in s:
			if c == chr(i):
				flg = 1
				break
		if flg == 0:
			print(chr(i))
			return
	print("None")
	return

if __name__ == '__main__':
	main()

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=[c for c in S]
C=list(set(A)^set(B))
A=sorted(set(C),key=A.index)
if len(A)==0:
    print("None")
else:
    print(A[0])

Solution (Python):
def conversion(a):
    num = ord(a) - 97
    return num
c = [] 
b = [] 
a = list(input())
ans = 0

for i in a:
    b.append(conversion(i))



for i in range(26):
    c.append(0)
for i in b:
    c[i] = 1


for i in c:
    if i == 0:
        break
    else:
        ans += 1



if ans == 26:
    print("None")
else:
    print(chr(ans + 97))

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

def li(): return map(int, stdin.readline().split())
def li_(): return map(lambda x: int(x)-1, stdin.readline().split())
def lf(): return map(float, stdin.readline().split())
def ls(): return stdin.readline().split()
def ns(): return stdin.readline().rstrip()
def lc(): return list(ns())
def ni(): return int(stdin.readline())
def nf(): return float(stdin.readline())

s = lc()

alpha = [chr(i) for i in range(ord("a"), ord("z")+1)]
lst = [0]*26

for i,a in enumerate(alpha):
    lst[i] = s.count(a)
    
exist = False
for i in range(26):
    if lst[i] == 0:
        print(alpha[i])
        exist = True
        break
        
if not exist:
    print("None")

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

Solution (Python):
s = list(input())
for i in range(97,123):
  if chr(i) not in s:
    print(chr(i))
    exit()
print("None")

Solution (Python):
s = set(list(input()))
for i in range(97,123):
  if chr(i) not in s:
    print(chr(i))
    exit()
print("None")

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

index=0
while index < len(listA):
    if listA[index] in alphaL:
        alphaL.remove(listA[index])
    index+=1

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

Solution (Python):
alphabets = 'a b c d e f g h i j k l m n o p q r s t u v w x y z'.split()
s = sorted(list(set(input())))
N = len(s)
i = 0
while(alphabets[i] in s):
    i += 1
    if(i == 26):
        break
if(i == 26):
    print('None')
else:
    print(chr(i+97))

Solution (Python):
S = set(input())
abc = "abcdefghijklmnopqrstuvwxyz"
if len(S) == len(abc):
  print(None)
for w in abc:
  if not w in S:
    print(w)
    break

Solution (Python):
s = input()

a = [c for c in "abcdefghijklmnopqrstuvwxyz"]


for i in range(len(a)):
    if a[i] not in s:
        print(a[i])
        exit(0)

print("None")

Solution (Python):
s=input()
res="None"
for i in range(97,97+26):
    if s.count(chr(i)) == 0:
        res=chr(i)
        break
print(res)

Solution (Python):
s = input()
sset = set(s)
ans = 'None'
abc = 'abcdefghijklmnopqrstuvwxyz'
for c in abc:
  if c not in sset:
    ans = c
    break
print(ans)

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

Solution (Python):
S=set(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'}
A=list(X-S)
A.sort()
if A==[]:
  print('None')
else:
  print(A[0])

Solution (Python):
S=set(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'}
A=list(X-S)
A.sort()
if A==[]:
  print('None')
else:
  print(A[0])

Solution (Python):
Alphabet = []
Alphabet.append("a")
Alphabet.append("b")
Alphabet.append("c")
Alphabet.append("d")
Alphabet.append("e")
Alphabet.append("f")
Alphabet.append("g")
Alphabet.append("h")
Alphabet.append("i")
Alphabet.append("j")
Alphabet.append("k")
Alphabet.append("l")
Alphabet.append("m")
Alphabet.append("n")
Alphabet.append("o")
Alphabet.append("p")
Alphabet.append("q")
Alphabet.append("r")
Alphabet.append("s")
Alphabet.append("t")
Alphabet.append("u")
Alphabet.append("v")
Alphabet.append("w")
Alphabet.append("x")
Alphabet.append("y")
Alphabet.append("z")
S = list(input())
S = list(set(S))
for i in range(len(S)) :
    for j in range(len(Alphabet)) :
        if S[i] == Alphabet[j] :
            Alphabet[j] = "-1"
if Alphabet.count("-1") == len(Alphabet) :
    print("None")
else :
    for k in range(len(Alphabet)) :
        if Alphabet[k] != "-1" :
            print(Alphabet[k])
            break

Solution (Python):
a=list(input())
b=[chr(i) for i in range(97, 97+26)]
for x in b:
  if not x in a:
     print(x)
     break
if len(set(a))==26:print('None')

Solution (Python):
S = input()
abc = 'abcdefghijklmnopqrstuvwxyz'
dic = {}
for s in abc:
    dic[s] = 0
for s in S:
    dic[s] += 1

for s in abc:
    if dic[s] == 0:
        print(s)
        exit()
print('None')

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


if len(aa) == 26:
    print('None')
else:
    for i in range(26):
        if alpha[i] not in a:
            print(alpha[i])
            break

Solution (Python):
S = list(input())
cnt = [0 for i in range(26)]
for i in range(len(S)):
    cnt[ord(S[i]) - 97] += 1
ans = -1
for i in range(26):
    if cnt[i] == 0:
        print(chr(i + 97))
        ans = 1
        break
if ans == -1:
    print("None")

Solution (Python):
S = input()
A = list('abcdefghijklmnopqrstuvwxyz')
flag = True
for i in A:
  if (i not in S):
    flag = False
    break
if flag:
  print('None')
else:
  print(i)

Solution (Python):
S = input()
alphabets = set(list('abcdefghijklmnopqrstuvwxyz'))
 
for s in S:
    alphabets.discard(s)
 
if len(alphabets) != 0:
    print(sorted(alphabets)[0])
else:
    print('None')

Solution (Python):
S = input()
X = [i for i in "abcdefghijklmnopqrstuvwxyz"]

S = [s for s in S]
S.sort()

for x in X:
    if x not in S:
        print(x)
        exit()

print("None")

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

for s in S:
    alphabets.discard(s)

if len(alphabets) != 0:
    print(sorted(alphabets)[0])
else:
    print('None')

Solution (Python):
from collections import defaultdict, Counter
from itertools import product, groupby, count, permutations, combinations
from math import pi, sqrt
from collections import deque
from bisect import bisect, bisect_left, bisect_right
from string import ascii_lowercase
from functools import lru_cache
import sys
sys.setrecursionlimit(10000)
INF = float("inf")
YES, Yes, yes, NO, No, no = "YES", "Yes", "yes", "NO", "No", "no"
dy4, dx4 = [0, 1, 0, -1], [1, 0, -1, 0]
dy8, dx8 = [0, -1, 0, 1, 1, -1, -1, 1], [1, 0, -1, 0, 1, 1, -1, -1]

def inside(y, x, H, W):
    return 0 <= y < H and 0 <= x < W


def ceil(a, b):
    return (a + b - 1) // b


def main():
    S = input()
    for c in ascii_lowercase:
        if c not in S:
            print(c)
            return
    print("None")


if __name__ == '__main__':
    main()

Solution (Python):
from collections import Counter
S=list(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 abc:
    if i not in S:
        print(i)
        exit()
print("None")

Solution (Python):
s = input()

num = {}
for ss in s:
    num[ss] = 1

ans = 'None'
for i in range(97, 97+26):
    if not chr(i) in num.keys():
        ans = chr(i)
        break
print(ans)

Solution (Python):
S = input()

dic = set()

for ch in S:
  dic.add(ch)

if len(dic) == 26:
  print("None")
  exit(0)

for ch in "abcdefghijklmnopqrstuvwxyz":
  if not ch in dic:
    print(ch)
    exit(0)

Solution (Python):
a = set("abcdefghijklmnopqrstuvwxyz") - set(input())
if a == set():
    print("None")
else:
    print(min(a))

Solution (Python):
import string
a = set(string.ascii_lowercase) - set(input())
if a == set():
    print("None")
else:
    print(min(a))

Solution (Python):
import string

s = input()
all = string.ascii_lowercase
rest = list(set(all) - set(s))
if rest == []:
    print('None')
else:
    rest.sort()
    print(rest[0])

Solution (Python):
s = set(input())
alphabet = set("abcdefghijklmnopqrstuvwxyz")
print("None" if len(s) == 26 else sorted(alphabet - s)[0])

Solution (Python):
s=input()
a='None'
for c in 'abcdefghijklmnopqrstuvwxyz':
    f=0
    for t in s:f|=c==t
    if not f:a=c;break
print(a)

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

res = "None"
for i in alphbet:
  if i not in S:
    res = i
    break
print(res)

Solution (Python):
import sys
import math
import collections
import itertools
import array
import inspect


sys.setrecursionlimit(10000)



def chkprint(*args):
    names = {
        id(v): k
        for k, v in inspect.currentframe().f_back.f_locals.items()
    }
    print(', '.join(
        names.get(id(arg), '???') + ' = ' + repr(arg) for arg in args))



def to_bin(x):
    return bin(x)[2:]


def li_input():
    return [int(_) for _ in input().split()]




dp = None


def main():
    alphabets = "abcdefghijklmnopqrstuvwxyz"

    S = input()

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

    for a in alphabets:
        if a not in S:
            print(a)
            return


main()

Solution (Python):
import sys
w=input()
alphabet=[chr(i) for i in range(97,97+26)]
for i in range (len(alphabet)):
    if w.count(alphabet[i])==0:
        print(alphabet[i])
        sys.exit()
print('None')

Solution (Python):
S = list( input())
alphabets = [ 0 for _ in range(26)]
for x in S:
    alphabets[ord(x)-97] = 1
ans = 'None'
for i in range(26):
    if alphabets[i] == 0:
        ans = chr(i+97)
        break
print(ans)

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

if S == test:
    print('None')

elif S[-1] == test[len(S)-1]:
    print(test[len(S)])

else:
    for num in range(len(S)):
        if test[num] != S[num]:
            print(test[num])
            break

Solution (Python):
S = sorted(list(set(list(input()))))
test = sorted('abcdefghijklmnopqrstuvwxyz')
ans = -1
if S == test:
    print('None')

elif S[-1] == test[len(S)-1]:
    print(test[len(S)])

else:
    for num in range(len(S)):
        if test[num] != S[num]:
            print(test[num])
            break

Solution (Python):

s = set(input())
diff = set('abcdefghijklmnopqrstuvwxyz')-s
if diff:
    print(sorted(diff)[0])
else:
    print('None')