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):
def solve():
    s = input()
    s = set(s)
    dic = set(["a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m", "n", "o", "p", "q", "r", "s", "t", "u", "v", "w", "x", "y", "z"])
    s = sorted(list(dic - s))
    if len(s) == 0:
        print("None")
        return
    print(s[0])


solve()

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

Solution (Python):
from collections import defaultdict
import sys,heapq,bisect,math,itertools,string,queue,datetime
sys.setrecursionlimit(10**8)
INF = float('inf')
mod = 10**9+7
eps = 10**-7
AtoZ = [chr(i) for i in range(65,65+26)]
atoz = [chr(i) for i in range(97,97+26)]

def inpl(): return list(map(int, input().split()))
def inpl_s(): return list(input().split())

S = input()

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

print('None')

Solution (Python):

import sys
S = input()
d = {}
for i in range(26):
    d[i] = 0
for i in range(len(S)):
    d[ord(S[i]) - ord('a')] = 1

for i in range(26):
    if d[i] == 0:
        print(chr(ord('a') + i))
        sys.exit()

print("None")

Solution (Python):


def main():
    s = input()
    abc = [chr(i) for i in range(ord('a'), ord('a')+26)]
    
    for i in abc:
        if i not in s:
            print(i)
            exit()
    print('None')

if __name__ == '__main__':
    main()

Solution (Python):
s = set(input())
a = "abcdefghijklmnopqrstuvwxyz"
for i in s:
  a = a.replace(i, "")
if a:
  print(a[0])
else:
  print("None")

Solution (Python):
l=sorted({chr(97+i) for i in range(26)}^{i for i in input()})
if not l:
  print("None")
else:
  print(l[0])

Solution (Python):
ans = [0 for i in range(26)]
for c in input():
    ans[ord(c)-ord('a')] += 1
if ans.count(0)==0:
    print('None')
else:
    print(chr(ord('a')+ans.index(0)))

Solution (Python):
s=sorted(list(set(input())))
flag=True
for i in range(26):
  a=i+97
  if s.count(chr(a))==0:
    flag=False
    break
print('None' if flag else chr(a))

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):
import sys
S = input()
target = 'abcdefghijklmnopqrstuvwxyz'
for chara in target:
  if chara in S:
    pass
  else:
    print(chara)
    sys.exit()
print('None')

Solution (Python):
S = input()
b =list('abcdefghijklmnopqrstuvwxyz')
c =[]
if len(set(S)) == len(b):
	print('None')
	exit()
for i in b:
    if i[0] not in S:
        print(i[0])
        exit()

Solution (Python):
s = raw_input()
ret = None
for x in "abcdefghijklmnopqrstuvwxyz"[::-1]:
	if x not in s:
		ret = x
print ret

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

albt = 'abcdefghijklmnopqrstuvwxyz'
for c in albt:
  if c not in s:
    print(c)
    exit()
print('None')

Solution (Python):
s = input()
from string import ascii_lowercase
t = set(ascii_lowercase)
a = t-set(s)
if len(a)==0:
  print("None")
else:
  print(sorted(a)[0])

Solution (Python):
S = input()
S_set = set(S)
alpha = set([chr(ord('a') + i) for i in range(26)])
ans = set(alpha - S_set)
ans = list(ans)
ans.sort()
if len(ans) >= 1:
	print(ans[0])
else:
	print("None")

Solution (Python):
al = list("abcdefghijklmnopqrstuvwxyz")
judge = "None"
S = list(input())
for i in range(26):
	if S.count(al[i]) == 0:
		judge = al[i]
		break

print(judge)

Solution (Python):
s=input()
ans=""
for i in range(97,123):
    ans+=chr(i)
for i in s:
    ans=ans.replace(i,"")
print(ans[0] if len(ans) else "None")

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

result = sorted(ans - s)
if result:
    print(result[0])
else:
    print("None")

Solution (Python):
N=[chr(i) for i in range(97,123)]
s=sorted(set(list(input())))
for i in N:
    if i  not in s:
        print(i)
        break
else:
    print("None")

Solution (Python):
import string
import sys
word = input()

for char in string.ascii_lowercase:
  if word.count(char) == 0:
    print(char)
    sys.exit()

print("None")
sys.exit()

Solution (Python):
"""
https://abc071.contest.atcoder.jp/tasks/abc071_b
"""
s=input()
e="abcdefghijklmnopqrstuvwxyz"
a=0
for i in range(len(e)):
    if s.find(e[i])==-1:
        print(e[i])
        break
    a=a+1
if a==len(e):
    print("None")

Solution (Python):

S=list(input())

ans=["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"]

S1=set(S)
ans1=set(ans)

ans_S=list(ans1-S1)
ans_S.sort()

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

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

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

Solution (Python):
import sys

S = set(input())
alpha = 'abcdefghijklmnopqrstuvwxyz'

for al in alpha:
    if al not in S:
        print(al)
        sys.exit()
print('None')

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

Solution (Python):
from heapq import heappop
S = list(input())
Q = [chr(ord('a') + i) for i in range(26)]
while S:
    l = S.pop()
    if l in Q:
        Q.remove(l)
if Q:
    print(heappop(Q))
else:
    print("None")

Solution (Python):


S = list(input())

sorted_list = sorted(list(set(S)))
abc_list = ["a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z"]

for str1 in abc_list:
    if str1 not in sorted_list:
        print(str1)
        exit()
print("None")

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

set_s = set(S)
set_all = set(list("abcdefghijklmnopqrstuvwxyz"))

set_diff = set_all - set_s

diff = []
for val in set_diff:
    diff.append(val)

ans = sorted(diff)

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

Solution (Python):

S = input()

alphabet = ['a', 'b', 'c', 'd','e','f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z', 'None']

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

Solution (Python):
from collections import Counter
st = Counter(raw_input())
for c in 'abcdefghijklmnopqrstuvwxyz':
  if st[c] == 0:
    print c
    break
else:
  print 'None'

Solution (Python):
from collections import Counter
S = list(input())
A = list("abcdefghijklmenopqrstuvwxyz")


def ans(S):
    for a in A:
        if a not in S:
            return a
    return "None"


print(ans(S))

Solution (Python):


s = input()
A = {}
for i in range(len(s)):
    if s[i] not in A:
        A[s[i]] = True

for i in range(26):
    character = chr(ord('a')+i)
    if character not in A.keys():
        print(character)
        exit()

print('None')

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

Solution (Python):
S = input()

al = list("abcdefghijklmnopqrstuvwxyz")

for i in range(len(S)):
    try:
        al.remove(S[i])
    except:
        pass

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

Solution (Python):
S = input()
S_list = list(set(S))
al = 'abcdefghijklmnopqrstucwxyz'
al_list = list(al)
S_list.sort()
flg = 1
for i in range(len(al_list)):
    if al_list[i] in S_list:
        pass
    else:
        print(al_list[i])
        flg = 0
        break
    
if flg == 1:
    print('None')

Solution (Python):
S = input()

alphabet_list = [
    "a",
    "b",
    "c",
    "d",
    "e",
    "f",
    "g",
    "h",
    "i",
    "j",
    "k",
    "l",
    "m",
    "n",
    "o",
    "p",
    "q",
    "r",
    "s",
    "t",
    "u",
    "v",
    "w",
    "x",
    "y",
    "z",
    ]

for s in S:
    if s in alphabet_list:
        alphabet_list.remove(s)
if len(alphabet_list) == 0:
    print("None")
else:
    print(alphabet_list[0])

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

Solution (Python):
S = input()

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 alphabet in alphabets:
    flag = 1
    for character in S:
        if character == alphabet:
            flag = 0
    if alphabet == "z" and flag == 0:
        print("None")
    elif flag == 1:
        print(alphabet)
        break

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

Solution (Python):
d = dict()
d['a'] = 1
d['b'] = 2
d['c'] = 3
d['d'] = 4
d['e'] = 5
d['f'] = 6
d['g'] = 7
d['h'] = 8
d['i'] = 9
d['j'] = 10
d['k'] = 11
d['l'] = 12
d['m'] = 13
d['n'] = 14
d['o'] = 15
d['p'] = 16
d['q'] = 17
d['r'] = 18
d['s'] = 19
d['t'] = 20
d['u'] = 21
d['v'] = 22
d['w'] = 23
d['x'] = 24
d['y'] = 25
d['z'] = 26

e = dict()
e[1] = 'a'
e[2] = 'b'
e[3] = 'c'
e[4] = 'd'
e[5] = 'e'
e[6] = 'f'
e[7] = 'g'
e[8] = 'h'
e[9] = 'i'
e[10] = 'j'
e[11] = 'k'
e[12] = 'l'
e[13] = 'm'
e[14] = 'n'
e[15] = 'o'
e[16] = 'p'
e[17] = 'q'
e[18] = 'r'
e[19] = 's'
e[20] = 't'
e[21] = 'u'
e[22] = 'v'
e[23] = 'w'
e[24] = 'x'
e[25] = 'y'
e[26] = 'z'

s =input()
lists = []
for i in range(len(s)):
    lists.append(d[s[i]])
lists.sort()
for i in range(1,28):
    if i != 27:
        if i not in lists:
            print(e[i])
            break
    else:
        print('None')

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

if len(set(s)) == 26:
    print("None")
else:
    alp = [chr(i) for i in range(97, 97+26)]
    for a in alp:
        if a not in sorted(set(s)):
            print(a)
            break

Solution (Python):
stringer = list("abcdefghijklmnopqrstuvwxyz")
strings = raw_input()

std = list(strings)

std.sort()
lister = {}

for j in range(len(stringer)):
    lister[stringer[j]]=0


for i in range(len(std)):
    count = lister[std[i]]
    lister[std[i]] = count + 1


hiter=0
for m in stringer:
    data = lister[m]
    if data == 0:
        hiter = m
        break

if hiter==0:
    print("None")
else:
    print(hiter)

Solution (Python):
S = input()

alphabet = "abcdefghijklmnopqrstuvwxyz"

ans = "None"
for i in range(len(alphabet)):
    if(alphabet[i] not in S):
        ans = alphabet[i]
        break
    
print(ans)

Solution (Python):
S = input()
alphabet = ["a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z"]

ans = "None"
for i in range(len(alphabet)):
    if(alphabet[i] not in S):
        ans = alphabet[i]
        break
    
print(ans)

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

Solution (Python):
rec = set(list("abcdefghijklmnopqrstuvwxyz"))
rec2 = set()
s = input()
for i in range(len(s)):
    rec2.add(s[i])

t = sorted(list(rec - rec2))
if len(t) == 0:
    print("None")
else:
    print(t[0])

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

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

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

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

Solution (Python):
S = raw_input()
lst = []
for i in range(0, 26):
	lst.append(True)

for i in range(0, len(S)):
	lst[ord(S[i])-97] = False

flag = True
for i in range(0, 26):
	if lst[i]:
		print chr(i+97)
		flag = False
		break

if flag:
	print 'None'

Solution (Python):
import string
S = set(input())
alpha = set(string.ascii_lowercase)
diff = alpha - S
if diff == set([]):
    print('None')
else:
    print(sorted(list(diff))[0])

Solution (Python):
a = "abcdefghijklmnopqrstuvwxyz"
S = input()
flg = 0
for i in a:
    if flg == 0 and i not in S:
        print(i)
        flg = 1
if flg == 0:
    print("None")

Solution (Python):
a = "abcdefghijklmnopqrstuvwxyz"
S = input()
flg = 0
for i in a:
    if flg == 0 and i not in S:
        print(i)
        flg = 1
if flg == 0:
    print("None")

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

Solution (Python):
a = input()

b='abcdefghijklmnopqrstuvwxyz'
ans ='None'
for i in b:
    if i not in a:
        ans = i
        print(ans)
        break
if ans == 'None':
    print('None')

Solution (Python):
s = input()
c_lst = [0 for i in range(26)]

for i in s:
    c_lst[ord(i)-97] += 1

for i in range(len(c_lst)):
    if c_lst[i] == 0:
        print(chr(i+97))
        break
else:
    print("None")

Solution (Python):
s=input()
j=0
for a in 'abcdefghijklmnopqrstuvwxyz':
    if a not in s:
        j=1
        print(a)
        break
if j != 1:
    print('None')

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

Solution (Python):
al = "abcdefghijklmnopqrstuvwxyz"
S1 = sorted(list(set(input())))
if (len(S1)==26):
    print("None")
else:
    for i in range (len(S1)):
        if al[i] != S1[i]:
            print(al[i])
            break
        elif i == len(S1)-1:
            print(al[len(S1)])
            break

Solution (Python):
S = set([str(x) for x in input()])
A = "abcdefghijklmnopqrstuvwxyz"
for a in A:
    if a not in S:
        print(a)
        exit(0)
print("None")

Solution (Python):
def a():
    S = input()

    for i in range(97,123):
        c = chr(i)
        if c not in S:
            print(c)
            return
    print("None")
    
a()

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

for x in ss:
    if x not in s:
        print(x)
        exit()
print('None')

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

for i in str:
    try:
        ans.remove(i)
    except:
        continue
try:
    print(ans[0])
except:
    print("None")

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

Solution (Python):
l = input()

ans = 'None'

for letter in 'abcdefghijklmnopqrstuvwxyz':
    if letter not in l:
        ans = letter
        break

print(ans)

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

for i in range(len(s)):
  a=cnt.index(s[i])
  ans[a]+=1
res=30
for i in range(26):
  if ans[i]==0:
    res=i
    break
if res==30:
  print("None")
else:
  print(cnt[res])

Solution (Python):
s = list(input())
s = sorted(s)
a_z = list('abcdefghijklmnopqrstuvwxyz')
flag = 1
for x in range(len(a_z)):
    if a_z[x] in s:
        flag = 1
    else:
        print(a_z[x])
        flag = 0
        break
if flag:
    print("None")

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

tt = list(sorted(list('abcdefghijklmnopqrstuvwxyz')))
u = []
for i in tt:
    if not (i in s):
        u.append(i)
        
uu = list(sorted(u))

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

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

tt = list(sorted(list('abcdefghijklmnopqrstuvwxyz')))
u = []
for i in tt:
    if not (i in s):
        u.append(i)
        
uu = list(sorted(u))

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

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

Solution (Python):
str_list = list(input())
import string
if set(str_list) == set(string.ascii_lowercase):
    print("None")
else:
    dif_list = sorted(list(set(string.ascii_lowercase) - set(str_list)))
    print(dif_list[0])

Solution (Python):
from string import ascii_lowercase

def first_not_in(s):
    for c in ascii_lowercase:
        if c not in s:
            return c
    return None

if __name__ == '__main__':
    print(first_not_in(input()))

Solution (Python):
import sys

stdin = sys.stdin

ni = lambda: int(ns())
na = lambda: list(map(int, stdin.readline().split()))
ns = lambda: stdin.readline().strip()

s = ns()
alfa = [chr(i) for i in range(97,123)]
ans = [chr(i) for i in range(97,123)]
for i in s:
    if i in alfa:
        if i in ans:
            ans.remove(i)

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

Solution (Python):
import string

s = input()
if len(set(s)) == 26: print('None')
else:
    for c in string.ascii_lowercase:
        if c not in s:
            print(c)
            break

Solution (Python):
import string
s = input()
dic = dict()
for c in s:
    dic[c] = 1
for i in range(len(string.ascii_lowercase)):
    c = string.ascii_lowercase[i]
    if not(c in dic):
        print(c)
        exit()
print('None')

Solution (Python):
ls = [chr(ord('a')+num) for num in range(26)]
s = input()
for x in range(len(ls)):
    if ls[x] not in s:
        print(ls[x])
        break
else:
    print("None")

Solution (Python):
import math
import sys

def getinputdata():

    
    array_result = []
    
    data = input()

    
    array_result.append(data.split(" "))

    flg = 1

    try:


        while flg:

            data = input()

            array_temp = []

            if(data != ""):
                
                array_result.append(data.split(" "))

                flg = 1

            else:

                flg = 0
    finally:


        return array_result





arr_data = getinputdata()

s = arr_data[0][0]

str="abcdefghijklmnopqrstuvwxyz"

for v in str:
    

    
    if v not in s:
        
        print(v)
        
        break
else:
    print("None")

Solution (Python):
S = input()
exist = set()
for i in S:
    exist.add(i)
for j in "abcdefghijklmnopqrstuvwxyz":
    if j not in exist:
        print(j)
        exit()
print("None")

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

Solution (Python):
S = set(input())
for alpha in "abcdefghijklmnopqrstuvwxyz":
    if alpha in S:
        pass
    else:
        print(alpha)
        break
else:
    print(None)

Solution (Python):
from collections import Counter
import string

S = input()
count = Counter(list(S))

found = False
for ch in string.ascii_lowercase:
    if count[ch] == 0:
        print(ch)
        exit()

if not found:
    print('None')

Solution (Python):
s = input()
a,num = "abcdefghijklmnopqrstuvwxyz",0
for i in range(26):
	if s.count(a[i])==0:
		print(a[i])
		break
	else:
		num += 1
if num==26:
	print("None")

Solution (Python):

a=set("qazwsxedcrfvtgbyhnujmikolp")
b=set(input())
print(sorted(a-b)[0] if len(a-b)>0 else 'None')

Solution (Python):
data=input()
temp=[chr(97+i) for i in range(26)]
for i in range(len(data)):
    temp[ord(data[i])-97]=0
cnt=26
for i in range(26):
    if temp[i]!=0:
        cnt=i
        break
if cnt<=25:print(temp[cnt])
else:print("None")

Solution (Python):
import sys;
print((sorted(set([chr(i) for i in range(ord('a'),ord('z')+1)])-set(sys.stdin.readline()))+[None])[0])

Solution (Python):
import sys
print((sorted(set([chr(i) for i in range(97,123)]) - set(sys.stdin.readline())) + [None])[0])

Solution (Python):
s=input()
for x in list('abcdefghijklmnopqrstuvwxyz'):
    if s.count(x)==0:print(x);exit()
print('None')

Solution (Python):





if __name__ == '__main__':
    S = sorted(set(list(input())))
    alphabets = [chr(ord('a') + i) for i in range(26)]

    if len(S) == 26:
        print('None')
    else:
        for alphabet in alphabets:
            if alphabet not in S:
                print(alphabet)
                break

Solution (Python):





if __name__ == '__main__':
    S = sorted(set(list(input())))
    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']

    if len(S) == 26:
        print('None')
    else:
        for alphabet in alphabets:
            if alphabet not in S:
                print(alphabet)
                break

Solution (Python):
S = input()
alpha = [chr(ord('a')+i) for i in range(26)]
for i in range(len(S)):
  if S[i] in alpha:
    alpha.pop(alpha.index(S[i]))
if len(alpha) > 0:
  print(alpha[0])
else:
  print('None')

Solution (Python):

s = input()
ans = set('abcdefghijklmnopqrstuvwxyz')-set(s)
if len(ans)==0:
    print('None')
else:
    print(min(ans))

Solution (Python):
s = input()
not_exists = []
for c in range(97, 123):
    if chr(c) not in s:
        not_exists.append(chr(c))
        print(chr(c))
        break
if len(not_exists) == 0:
    print('None')

Solution (Python):
s=input()
exist=[False for i in range(26)]

for c in s:
	exist[ord(c)-ord('a')]=True
for i in range(26):
	if not exist[i]:
		print(chr(ord('a')+i))
		break
else:
	print("None")

Solution (Python):
S = input()

s_char = [c for c in S]
alphas = [chr(i) for i in range(97, 97 + 26)]

for alpha in alphas:
    if alpha not in s_char:
        print(alpha)
        exit()
print("None")

Solution (Python):
alphabet = "abcdefghijklmnnopqrstuvwxyz"
def solve():
    s = input()
    for i in alphabet:
        if i not in s:
            print(i)
            break
    else:
        print("None")
solve()

Solution (Python):
import string
S = str(input())
a = set(S)
b = set(string.ascii_lowercase)
c = b - a
if len(list(c)) == 0:
    print(None)
else:
    print(sorted(list(c))[0])

Solution (Python):
s = input()
ch = {chr(i) for i in range(ord('a'), ord('z')+1)}
ss = set()
for i in range(len(s)): ss.add(s[i])
tmp = sorted(list(ch ^ ss))
print(tmp[0] if len(tmp) else "None")

Solution (Python):
S=sorted(list(set(list(input()))))
alph=["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"]
sum=0
A="None"
for i in range(len(S)):
    if S[i]!=alph[i]:
        A=alph[i]
        break
if A=="None" and len(S)<26:
    A=alph[i+1]

print(A)

Solution (Python):
s=input()
l=[]
for i in s:
    l.append(i)
l.sort()
li=[]
for i in range(len(l)-1):
    if(l[i]!=l[i+1]):
        li.append(l[i])
li.append(l[-1])
alph="abcdefghijklmnopqrstuvwxyz"
for i in li:
    for j in range(len(alph)):
        if(i==alph[j]):
            alph=alph.replace(i,"")
            break
if(len(alph)==0):
    print("None")
else:
    print(alph[0])

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

Solution (Python):
s = input()
ls = []
nflag = False
for i in range(len(s)):
    ls.append(s[i])

for i in range(26):
    if not chr(i+97) in ls:
        print(chr(i+97))
        nflag = True
        break

if not nflag:
    print('None')

Solution (Python):
S=set(input())
Sl=list(S)
Sl.sort()
moji=["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"]
T="None"
for i in range(len(Sl)):
    if Sl[i]!=moji[i]:
        T=moji[i]
        break

if T=="None" and len(Sl)<26:
    T=moji[i+1]

print(T)

Solution (Python):
S = input()
ABC = [0 for i in range(26)]
for i in S:
    ABC[ord(i)-97] += 1
flag = 0
for i in range(26):
    if ABC[i] == 0 and flag == 0:
        print(chr(i+97))
        flag = 1
if flag == 0:
    print("None")

Solution (Python):
lis = ["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"]
n = list(input())
k = 0
while 1:
  if lis[k] not in n:
    print(lis[k])
    break
  k += 1

Solution (Python):
import string
xs = input()


def foo(xs):
    a = string.ascii_lowercase
    cnt = [0] * len(a)
    for x in xs:
        cnt[ord(x) - ord('a')] += 1

    for i, c in enumerate(cnt):
         if c == 0:
            print(a[i])
            return
    print("None")
    return


foo(xs)

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

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

print("None")

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

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

Solution (Python):
dic = 'abcdefghijklmnopqrstuvwxyz'
s = input()
import sys
for d in dic:
    if d not in s:
        print(d)
        sys.exit(0)
print('None')

Solution (Python):

if __name__ == '__main__':

    S = input()
    flag = True
    for i in range(26):
        target = chr(ord('a') + i)
        if S.find(target) == -1:
            print(target);
            flag = False
            break

    if flag :
        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=list(input())

al=list('abcdefghijklmnopqrstuvwxyz')

r=list(set(al)-set(s))
r.sort()

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

Solution (Python):
s=list(set(input()))
test=["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"]
test.sort()
s.sort()
if len(s)==len(test):
    print("None")
    exit()

i=0
while True:
    if len(s)<=i:
        print(test[i])
        break
        
    if test[i]!=s[i]:
        print(test[i])
        break
    i+=1

Solution (Python):
import string

s = input()
for c in string.ascii_lowercase:
  if c not in s:
    print(c)
    exit()
print('None')

Solution (Python):
s=input()
a=[chr(x) for x in range(ord('a'),ord('z')+1)]
for k in a:
	if k not in s:
		print(k)
		exit()
print('None')

Solution (Python):
print((sorted(set("aquickbrownfoxjumpsoverthelazydog")-set(input()))+["None"])[0])

Solution (Python):


s = list(input())

answer = "None"
for n in range(ord("a"), ord("a") + 26):
    ch = chr(n)
    if not ch in s:
        answer = ch
        break
print(answer)

Solution (Python):
alphabet = "abcdefghijklmnnopqrstuvwxyz"

def solve():
	S = input()
	for c in alphabet:
		if c not in S:
			print(c)
			exit()
	print("None")
	exit()
			
			
solve()

Solution (Python):
S=input()
ans='None'
for c in range(26):
  if chr(97+c) not in S:
    ans = chr(97+c)
    break
print(ans)

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):
alphabet = "abcdefghijklmnopqrstuvwxyz"

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

Solution (Python):
alp = "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"
alp = list(map(str, alp.split()))
s = input()
for i in range(len(s)):
    for j in range(26):
        if(s[i] == alp[j]):
            alp[j] = "?"
while("?" in alp):
    alp.remove("?")
if(len(alp) == 0):
    print("None")
else:
    print(alp[0])

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

Solution (Python):
S = 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 = raw_input()
f = False
for x in 'abcdefghijklmnopqrstuvwxyz':
	if x not in s:
		print x
		f = True
		break
if not f:
	print 'None'

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

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

Solution (Python):
s = raw_input()
f = False
for x in 'abcdefghijklmnopqrstuvwxyz':
	if x not in s:
		print x
		f = True
		break
if not f:
	print 'None'

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

Solution (Python):
val = list(input())
alp = ["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"]

val2 = set(val)
for i in val2:
    if i in alp:
        alp.remove(i)
try:
    print(alp[0])
except IndexError:
    print("None")

Solution (Python):
s=input()
d=[0 for i in range(26)]
for i in range(len(s)):
  d[ord(s[i])-ord("a")]+=1
ss="None"
for i in range(26):
 if d[i]==0:
    ss=chr(i+ord("a"))
    break
print(ss)

Solution (Python):
import string
 
S = set(list(input()))
ans = set(list(string.ascii_lowercase)) - S
if len(ans) == 0:
    print("None")
    exit()
ans = sorted(list(ans))[0]
print(ans)

Solution (Python):
S = input()

N = "None"
count = 0
app = [0] * 26
for i in range(len(S)):
    app[ord(S[i]) - ord("a")] = 1

for i in range(26):
    if app[i] == 1:
        count += 1
        if count == 26:
            print(N)

    if app[i] == 0:
        print (chr(i + ord("a")))
        break

Solution (Python):
import string

S = set(list(input()))
ans = set(list(string.ascii_lowercase)) - S
if len(ans) == 0:
    print("None")
    exit()
ans = sorted(list(ans))[0]
print(ans)

Solution (Python):
S = raw_input()
for c in [chr(ord("a") + i) for i in xrange(26)]:
    if c in S:
        continue
    else:
        print c
        break
else:
    print "None"

Solution (Python):



import string

def main():
    S = set(input())
    T = set(string.ascii_lowercase)

    T -= S
    if not T:
        print("None")
    else:
        print(sorted(T)[0])

if __name__ == "__main__": main()

Solution (Python):
abc = 'abcdefghijklmnopqrstuvwxyz'
S = input()
for x in abc:
    if not x in S:
        print(x)
        break
else:
    print('None')

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

Solution (Python):
if __name__ == '__main__':
    S = input()
    dic = [chr(i) for i in range(97, 123)]
    j=0
    for i in range(len(dic)):
        if dic[i] not in S:
            j = 1
            print(dic[i])
            break
    if j == 0:
        print('None')

Solution (Python):
import string
S = input()
for a in string.ascii_lowercase:
    if a not in S:
        print(a)
        exit(0)
print('None')

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

Solution (Python):
S = input()
alphabet = "abcdefghijklmnopqrstuvwxyz"
found = False
for i in alphabet:
  if i not in S:
    print(i)
    found = True
    break

if not found:
    print('None')

Solution (Python):
S = input()
alphabet = "abcdefghijklmnopqrstuvwxyz"
found = [i for i in alphabet if i not in S]
found.sort()
if len(found) == 0:
  print('None')
else:
  print(found[0])