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 = list(input().strip())
s.sort()
i = 0
for c in "abcdefghijklmnopqrstuvwxyz!":
    if i < len(s) and s[i] == c:
        while i < len(s) and s[i] == c:
            i += 1
    else:
        if c != "!":
            print(c)
        else:
            print("None")
        raise SystemExit

Solution (Python):
S = set(input())
sort = sorted(S)
e = 'no'
for i in range(97, 123):
    if chr(i) not in sort:
        print(chr(i))
        e = 'yes'
        break
if e == 'no':
    print('None')

Solution (Python):
import math,string,itertools,fractions,heapq,collections,re,array,bisect,sys,random,time,copy,functools
 
sys.setrecursionlimit(10**7)
inf = 10**20
gosa = 1.0 / 10**10
mod = 10**9 + 7
 
def LI(): return [int(x) for x in sys.stdin.readline().split()]
def LI_(): return [int(x)-1 for x in sys.stdin.readline().split()]
def LF(): return [float(x) for x in sys.stdin.readline().split()]
def LS(): return sys.stdin.readline().split()
def I(): return int(sys.stdin.readline())
def F(): return float(sys.stdin.readline())
def S(): return input()
 
 
def main():
    c = collections.Counter(S())
    for k in string.ascii_lowercase:
        if c[k] == 0:
            return k
    return 'None'
 
print(main())

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

Solution (Python):

s = list(input().rstrip())
alpha = list(map(chr, range(ord('a'), ord('z') + 1)))

for a in alpha:
    if a not in s:
        print(a)
        break
else:
    print("None")

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

Solution (Python):
from collections import defaultdict
from itertools import product, groupby
from math import pi
from collections import deque
from bisect import bisect, bisect_left, bisect_right
from string import ascii_lowercase
INF = 10 ** 10


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



if __name__ == '__main__':
    main()

Solution (Python):
s=list(input())
L=[]
for t in range(0,26):
    if chr(97+t) not in s:
        a=chr(97+t)
        L.append(a)
        
if not L:
    print("None")
else:
    print(L[0])

Solution (Python):
all_char = map(chr,range(97,123))
all_set = set(all_char)
input_set = set(input())
print(min(all_set-input_set or{"None"}))

Solution (Python):
s = input()
swi = 0
lst = ["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 (lst[i] in s) == False:
        print(lst[i])
        swi = 1
        break
if swi == 0:
    print("None")

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

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

Solution (Python):
print(min(set(chr(97+i)for i in range(26))-set(input())or["None"]))

Solution (Python):
print((lambda s:next((c for c in map(chr,range(97,123))if c not in s),"None"))(input()))

Solution (Python):
print(min([set(chr(97+i)for i in range(26))-set(input())or["None"]][0]))

Solution (Python):
print([sorted(set(chr(97+i)for i in range(26))-set(input()))or["None"]][0][0])

Solution (Python):
x=set(chr(97+i) for i in range(26))-set(input())
print("None" if not x else min(x))

Solution (Python):
print((lambda x:"None" if not x else min(x))(set(chr(97+i) for i in range(26))-set(input())))

Solution (Python):
print((lambda x:"None" if not x else x[0])(sorted(set(chr(97+i) for i in range(26))-set(input()))))

Solution (Python):
x=sorted(set(chr(97+i) for i in range(26))-set(input()))
print("None" if not x else x[0])

Solution (Python):
print((lambda x:"None" if not x else x[0])(sorted(set(chr(97+i) for i in range(26))-set(list(input())))))

Solution (Python):
print((lambda x:"None" if not x else x[0])(sorted(list(set(chr(i) for i in range(97,97+26))-set(input())))))

Solution (Python):
print((lambda x:"None" if not x else x[0])(sorted(list(set([chr(i) for i in range(97, 97+26)]) - set(list(input()))))))

Solution (Python):
x=set([chr(i) for i in range(97,97+26)])-set(list(input()))
print("None" if not x else sorted(list(x))[0])

Solution (Python):
print((lambda x:"None" if not x else sorted(list(x))[0])(set([chr(i) for i in range(97,97+26)])-set(list(input()))))

Solution (Python):

"""
"""


def main():
    alphabets = 'abcdefghijklmnopqrstuvwxyz'
    text = input().strip()

    result = 'None'
    for c in alphabets:
        if c not in text:
            result = c
            break
    print(result)


if __name__ == '__main__':
    main()

Solution (Python):
bun=input()
mm="abcdefghijklmnopqrstuvwxyz"
for x in range(len(mm)):
    if mm[x] not in bun:
        print(mm[x])
        break
else:
    print("None")

Solution (Python):
import string


def main():
    cs = set(input())
    print(solve(cs))


def solve(cs):
    for c in string.ascii_lowercase:
        if c not in cs:
            return c
    return 'None'


if __name__ == '__main__':
    main()

Solution (Python):
S = input()

for i in range(97, 97 + 26):
    if chr(i) not in S:
        print(chr(i))
        exit()

print('None')

Solution (Python):
S = input()

a = 0
for i in range(97, 97 + 26):
    if chr(i) not in S:
        print(chr(i))
        a = 1
        break

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

Solution (Python):
import collections
s = collections.Counter('abcdefghijklmnopqrstuvwxyz')
l = collections.Counter(input())
ret = 'None'
for el in sorted((s+l).items()):
  if el[1] == 1:
    ret = el[0]
    break
print(ret)

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

for alp in alphabet:
    if alp not in S:
        print(alp)
        break
else:
    print("None")

Solution (Python):
s = input()

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


for v in l:
	if v in s:
		if v == "z":
			print("None")
			break
	else:
		print(v)
		break

Solution (Python):
def solve(): 
    alphabet = []
    for i in range(26):
        alphabet.append(chr(97 + i))
    s = list(input())

    for j in s:
        if j in alphabet:
            alphabet.remove(j)
        else:
            pass

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


solve()

Solution (Python):
S = 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):
import sys
line = raw_input()
res = dict()
for i in line:
    res[i] = 1
    if len(res) == 26:
        print 'None'
        sys.exit(0)
for i in 'abcdefghijklmnopqrstuvwxyz':
    if res.get(i, None) is None:
        print i
        sys.exit(0)

Solution (Python):
S = set(raw_input())
T = set([chr(i) for i in range(ord('a'), ord('z')+1)])
U = T - S
print "None" if len(U) == 0 else sorted(list(U))[0]

Solution (Python):
import sys

str = sys.stdin.read()
words = "abcdefghijklmnopqrstuvwxyz"
wordslist= list(words)
list2 = list(words)
try:
    for item in wordslist:
        iss = str.find(item)
        if iss != -1:
            list2.remove(item)
except Exception:
    pass

if len(list2) == 0:
    print("None")
else:
    print(list2[0])

Solution (Python):
import sys

alphabet = "abcdefghijklmnopqrstuvwxyz"
word = raw_input()
found = 0

for letter in alphabet:
    if letter not in word:
        print letter
        found = 1
        break

if found == 0:
    print "None"

Solution (Python):

S = input()
res = "None"

for c in range(26):
    if chr(ord('a') + c) not in S:
        res = chr(ord('a') + c)
        break
print(res)

Solution (Python):
import string
import sys

S = input()
l = string.ascii_lowercase

for c in l:
  there = False
  for cs in S:
    if c == cs:
      there = True
  if there == False:
    print(c)
    sys.exit()

print("None")
sys.exit()

Solution (Python):
S = str(input())
alphabet = "abcdefghijklmnopqrstuvwxyz"

flag = [0] * 26
for i in range(26):
    if alphabet[i] in S:
        flag[i] = 1

for i in range(26):
    if flag[i] == 0:
        print(alphabet[i])
        break

if flag == [1] * 26:
    print("None")

Solution (Python):
import math,string,itertools,fractions,heapq,collections,re,array,bisect,sys,random,time,copy,functools

sys.setrecursionlimit(10**7)
inf = 10**20
gosa = 1.0 / 10**10
mod = 10**9 + 7

def LI(): return [int(x) for x in sys.stdin.readline().split()]
def LI_(): return [int(x)-1 for x in sys.stdin.readline().split()]
def LF(): return [float(x) for x in sys.stdin.readline().split()]
def LS(): return sys.stdin.readline().split()
def I(): return int(sys.stdin.readline())
def F(): return float(sys.stdin.readline())
def S(): return input()


def main():
    c = collections.Counter(S())
    for k in string.ascii_lowercase:
        if c[k] == 0:
            return k
    return 'None'

print(main())

Solution (Python):
al = set([chr(97 + i) for i in range(26)])
inp = set([x for x in input()])

mal = al.difference(inp)

if len(mal):
    print(chr(min(map(ord,mal))))
else:
    print("None")

Solution (Python):



import sys

def strchr(string, char):
	for token in string:
		if (token == char):
			return 1
	return 0

S = raw_input()

tmp = chr(ord('a'))
for i in range(0, 26):
	if (strchr(S, tmp) == 0):
		print tmp
		sys.exit()
	tmp = chr(ord(tmp) + 1)

print "None"

Solution (Python):


s = set(input())
ab = {chr(i) for i in range(97,97+26)}
ab -= s
if len(ab) == 0:
    print("None")
else:
    print(min(ab))

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

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

Solution (Python):
from string import ascii_lowercase

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

if len(s) == 26:
    print("None")
else:
    print(sorted(t - s)[0])

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

for x in l:
    if not x in S:
        print(x)
        exit()
        
print('None')

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

for x in l:
    if not x in S:
        print(x)
        break
    else:
        if x == 'z':
            print('None')

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

none_alpha = ["None"]

for i in range(26):
    if alphabet[i] in none_alpha:
        continue;
    a = word.find(alphabet[i])
    if a == -1:
        if "None" in none_alpha:
            none_alpha.remove("None")
        none_alpha.append(alphabet[i])

print(none_alpha[0])

Solution (Python):
S = 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"]

i = 0
while i in range(len(alphabets)):
    if alphabets[i] in S:
        i += 1
    else:
        print(alphabets[i])
        break
else:
    print ("None")

Solution (Python):
import string

s = input()
alp = string.ascii_lowercase

def search(s, c):
    for i in s:
        if i == c:
            return 1
    return 0

f = 0
for i in alp:
    if not search(s, i):
        print (i)
        f = 1
        break;
if not f:
    print ("None")

Solution (Python):
s = input()

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

flag = [False for i in range(26)]

for c in s:
    flag[ord(c) - ord("a")] = True


def pr():
    for i in range(26):
        if not flag[i]:
            print(alp[i])
            return

    print('None')
    return

pr()

Solution (Python):
S = input()
ls = [chr(i) for i in range(ord("a"),ord("z")+1)]
for c in S:
    if c in ls:
        ls.remove(c)
if ls==[]:
    print("None")
else:
    print(ls[0])

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

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

X = set('abcdefghijklmnopqrstuvwxyz')               

S=X-S                     

print("None" if S==set() else min(S))

Solution (Python):
import string
s = list(input())
a = list(string.ascii_lowercase)
l = []
for index, i in enumerate(a):
    if i not in s:
        print(i)
        break
else:
    print(None)

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

if __name__ == "__main__":
    main()

Solution (Python):

a = list('abcdefghijklmnopqrstuvwxyz')
S = list(input())
b = list(set(S))
for tmp in b:
    a.pop(a.index(tmp))
if len(a) == 0:
    print("None")
else:
    print(a.pop(0))

Solution (Python):
import string

S = input()

X = set(string.ascii_lowercase) - set(S)
if X:
    print(sorted(list(X))[0])
else:
    print(None)

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

Solution (Python):
s=input()
ans="None"
import string
for i in string.ascii_lowercase:
    if i not in s:
        ans=i
        break
print(ans)

Solution (Python):
s=input()
a=ord('a')
z=ord('z')
flag=False
for i in range(a,z+1):
    c=chr(i)
    if not c in s:
        flag=True
        print(c)
        break

if not flag:
    print('None')

Solution (Python):
s = raw_input()
for i in range(26):
    c = chr(ord('a')+i)
    if c in s:
        continue
    else:
        print c
        break
else:
    print 'None'

Solution (Python):

import sys

s=raw_input()
l=[False for i in xrange(26)]
for i in xrange(len(s)):
 temp=ord(s[i])-97
 l[temp]=True
if len(set(s))==26:
 print 'None'
 sys.exit()
for i in xrange(26):
 if not l[i]:
  print chr(i+97)
  break

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

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):
import string


def main():
    s = input()
    for c in string.ascii_lowercase:
        if c not in s:
            print(c)
            break
    else:
        print("None")

if __name__ == "__main__":
    main()

Solution (Python):
import string

a = input()

f = False
for i in string.ascii_lowercase:
    if i not in a:
        print(i)
        f = True
        break

if not f:
    print("None")

Solution (Python):
import string
import sys
from collections import Counter




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


def read_str_list():
    return input().split()


def read_int():
    return int(input())


def read_str():
    return input()


def main():
    a = list(string.ascii_lowercase)
    res = None
    s = read_str_list()
    l = list(s)
    l.sort()
    s = ''.join(l)

    c = Counter(s)
    for i in a:
        if c[i] == 0:
            res = i
            break

    print(res)


main()

Solution (Python):
s = list(input())
d = {}
for c in s:
    d[c] = True
ans = "None"
for i in range(26):
    c = chr(ord('a') + i)
    if c not in d:
        ans = c
        break

print(ans)

Solution (Python):
s=raw_input()
a=[0 for i in range(26)]
p=-1
for i in range(len(s)):
    a[ord(s[i])-97]=1
for i in range(26):
    if a[i]==0:
        print chr(i+97)
        exit()
print "None"

Solution (Python):
S = set(map(ord,input()))
alphabet = set(map(ord,'abcdefghijklmnopqrstuvwxyz'))

x = alphabet - S
try:
    print(chr(min(x)))
except:
    print('None')

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

Solution (Python):
s=input()
char=['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']
check=False
for c in char:
    if not c in s:
        print(c,end='')
        check=True
        break
if  check==False:
    print('None',end='')

Solution (Python):
S = str(input())
list_S = []

for i in range(len(S)):

    list_S.append(S[i:i+1])



alf = 'abcdefghijklmnopqrstuvwxyz'
list_alf = []
for i in range(len(alf)):
    list_alf.append(alf[i:i+1])

count = 0
conti = 0

for i in range(len(alf)):
    count = list_S.count(list_alf[i])
    if count == 0:
        print (list_alf[i])
        break
    conti += 1


if conti >= 26:
    print ('None')

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

Solution (Python):
st = input()
hash_set = set()
for item in st:
    hash_set.add(item)

a = "abcdefghijklmnopqrstuvwxyz"

flag = False
for item in a:
    if not item in hash_set:
        print(item)
        flag = True
        break

if flag == False:
    print("None")

Solution (Python):
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']

n = 0

for i in range(26):

	if alpha[n] not in S:
		print(alpha[n])
		break
	if n == 25:
		print("None")
		break
		
	n += 1

Solution (Python):
S = raw_input()
ss = set(S)
ans = set("abcdefghijklmnopqrstuvwxyz").difference(ss)

if len(ans): print(min(ans))
else: print("None")

Solution (Python):
s = input()
s_list = list(s)
s_set = set(s_list)
abc_list = [chr(i) for i in range(97, 97+26)]
if len(s_set) == 26:
    print("None")
else:
    for a in abc_list:
        if a in s_set:
            pass
        else:
            print(a)
            break

Solution (Python):
S=input()
check=True
 
for i in range(0,26):
    if S.find(chr(i+97))==-1:
        print(chr(i+97))
        check=False
        break


print("None" if check else "")

Solution (Python):
a = input()

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

flag = True;
for c in alpha:
    if c not in a:
        print(c)
        flag = False
        break

if flag:
    print("None")

Solution (Python):
S = input()
countList = []
alphabetList = []
for i in range(ord("a"),ord("z")+1):
    alphabetList.append(chr(i))
    countList.append(S.count(chr(i)))
if min(countList) != 0:
    print("None")
else:
    print(alphabetList[countList.index(0)])

Solution (Python):

s=input()
s=list(s)
nums=[]
for i in s: nums.append(ord(i))

flag=True
for i in range(97,123):
    if i in nums: pass
    else: print(chr(i));flag=False; break
if flag: print("None")

Solution (Python):
S = input()
l = set(list(S))
l = {'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z'} - l
l = list(l)
l.sort()
print(l[0] if(len(l)) else "None" )

Solution (Python):
st = list(input())
ls = list(set(st))
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']
set_ab = set(abc) - set(ls)
ans = list(set_ab)
ans.sort()
if ans:
    print(ans[0])
else:
    print("None")

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

Solution (Python):
def b():
    alphabets = 'abcdefghijklmnopqrstuvwxyz'
    s = input()
    
    diff = set(alphabets) - set(s)
    try:
        print( sorted(list(diff))[0] )
    except:
        print('None')

b()

Solution (Python):
import math
def search(x,n):
	curPos=0
	log2=int(math.log(n,2))
	newPos=0
	if s[curPos]==x:
		return True
	while log2 >=0:
		newPos=curPos + (1 << log2)
		if newPos < n and s[newPos]<=x:
			curPos=newPos
		if s[curPos]==x:
			return True
		log2-=1
	return False

s=input()
s=list(s)
s.sort()
n=len(s)
s_str=[chr(x) for x in range(97,123)]
Found=False
for i in s_str:
	res=search(i,n)
	if not res:
		print(i)
		Found=True
		break
if not Found:
	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"]
S = list(set(S))
S.sort()

for i in range(len(S)):
    if S[i] != alpha[i]:
        print(alpha[i])
        break
    elif i == 25:
        print("None")
    elif i == len(S) - 1:
        print(alpha[i+1])

Solution (Python):
from collections import Counter
S = input()

counter = dict(Counter(S))
counter = sorted(counter) 
alphabets = "abcdefghijklmnopqrstuvwxyz"

found = False
for letter in alphabets:
    if letter in counter:
        continue
    else:
        found = True
        print(letter)
        break

if found is not True:
    print("None")

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

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

Solution (Python):
letters = [chr(i) for i in range(97,123)]
string  = input()
for j in range(len(string)):
    if string[j] in letters:
        letters.remove(string[j])
    else:
        continue
if letters:
    print(letters[0])
else:
    print("None")

Solution (Python):

S=map(str,raw_input())
S.sort()
flag=1
SS=[]
for i in S:
    SS.append(ord(i))
for i in range(ord("a"),ord("z")+1):
    if not i in SS:
        ans=chr(i)
        print ans
        flag=0
        break
if flag==1:
    print "None"

Solution (Python):
def main():
    S = input()
    alphabet = "abcdefghijklmnopqrstuvwxyz"
    
    
    
    dic = str.maketrans("", "", S)
    ans = alphabet.translate(dic)
    if(len(ans) > 0):
        print(ans[0])
    else:
        print("None")
    
    
if __name__=="__main__":
    main()

Solution (Python):
word_list = list(input().lower())
alphabet_uniq = list(set(word_list))
alphabet_uniq.sort()

alphabet_uniq.append('END')
if len(alphabet_uniq) != 27:
    for i in range(0, len(alphabet_uniq)):
        if chr(i + 97) != alphabet_uniq[i]:
            print(chr(i + 97))
            break
else:
    print(None)

Solution (Python):
s = input()

a = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z']

for c in s:
    if c in a:
        a.remove(c)
if len(a) > 0:
    print(a[0])
else:
    print("None")

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

for i in range(97,123):
    if(x.find(chr(i))==-1):
        print(chr(i))
        quit()

print('None')


quit()

Solution (Python):
a = raw_input().strip()
al = 'abcdefghijklmnopqrstuvwxyz'
for x in al:
	try:
		i = a.index(x)
	except :
		print(x)
		exit()
print('None')

Solution (Python):
import sys

DICT = ['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']

d = set(DICT)
s = set(list(sys.stdin.readline().strip()))

z = sorted(list(d-s))

if len(z) >= 1:
    print(z[0])
else:
    print('None')

Solution (Python):


S = input()
CHR = [chr(i) for i in range(97, 97+26)]

judge = 1
i = 0

while(judge > 0 and i <= 25):
    judge = S.count(CHR[i])
    i = i+1

if(judge==0):
    print(CHR[i-1])
else:
    print("None")

Solution (Python):
s = input()

l = list(s)




l_abc = list("abcdefghijklmnopqrstuvwxyz")

l = list(set(l_abc) - set(l))
l.sort()




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

Solution (Python):
string = ['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 =[d for d in input()]
find = 0
i=0
found = False
while i<26 and not found:
    if string[i] in s :
        found= False
    else:
        find =string[i]
        found=True
    i +=1

if found:
    print(find)
else:
    print('None')

Solution (Python):
import sys
S = input()
a = 'abcdefghijklmnopqrstuvwxyz'
a = 'abcdefghijklmnopqrstuvwxyz'
for aa in a:
    if aa in S:
        pass
    else:
        print(aa)
        sys.exit()
print('None')

Solution (Python):
S = input()

X = 'abcdefghijklmnopqrstuvwxyz'

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

print('None')

Solution (Python):
if __name__ == '__main__':
    char = list('abcdefghijklmnopqrstuvwxyz')
    S = input()
    for i in range(len(S)):
        if S[i:i+1] in char:
            char.remove(S[i:i+1])
    if len(char) == 0:
        print('None')
    else:
        print(char[0])

Solution (Python):
abc='abcdefghijklmnopqrstuvwxyz'
s=raw_input().strip()
isi=set()
for l in s:
    if l not in isi:
        isi.add(l)
for l in abc:
    if l not in isi:
        print l
        break
else:
    print 'None'

Solution (Python):
s2 = "abcdefghijklmnopqrstuvwxyz"
s2 = set(s2)
s = set(raw_input())
if len(s) == 26:
    print "None"
a = list(s2 -s)
if len(a) >0:
  a.sort()
  print a[0]

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

Solution (Python):
import sys
S=list(raw_input())



A=list('abcdefghijklmnopqrstuvwxyz')




for i in range(len(A)):
	
	for cnt,v in enumerate(S):
		
		
		if A[i]==v:
			if A[i]=='z':
				print 'None'
				break
			else:
				break
		if cnt==len(S)-1:
			print A[i]
			sys.exit()

Solution (Python):
s = list(set(input()))
abc = "abcdefghijklmnopqrstuvwxyz"
s.sort()
if len(s)==26: print("None")
else:
  for a in abc:
    if not(a in s):
      print(a)
      break

Solution (Python):
alphabet_list = [0 for x in range(ord('a'),ord('z')+1)]

S = list(raw_input())

for x in S:
    alphabet_list[ord(x) - ord('a')] += 1

print_flag = False
for index in range(0,ord('z')-ord('a')+1):
    if alphabet_list[index] == 0:
        print(chr(index+ord('a')))
        print_flag = True
        break

if not(print_flag):
    print('None')

Solution (Python):
a_list=("abcdefghijklmnopqrstuvwxyz")
word=(raw_input())
flag=0
out="None"
for i in a_list:
    for n,k in enumerate(word):
        if i==k:
            break
        if n==len(word)-1:
            out=i
            flag=1
    if flag==1:
        break
print(out)

Solution (Python):
def B_NotFound(lst):
    comparison = 'abcdefghijklmnopqrstuvwxyz'
    lst = sorted(set(lst))  

    string = [''] * len(comparison)
    for i in range(len(lst)):
        string[i] = lst[i]

    for i in range(len(string)):
        if string[i] != comparison[i]:
            return comparison[i]
    return 'None'

s = input()
print(B_NotFound(s))

Solution (Python):
def search(code):
    for i in range(len(code)):
        if code[i] == 0:
            return i
    else:
        return -1

s = raw_input()

code = [0 for i in range(26)]

for i in range(len(s)):
    key = ord(s[i]) - 97
    code[key] += 1

rt = search(code)

if rt >= 0:
    print chr(rt + 97)
else:
    print "None"

Solution (Python):
s = input()
arr = "abcdefghijklmnopqrstuvwxyz"
ans = []
for word in arr:
    if word in s:
        pass
    else:
        ans.append(word)
if len(ans) == 0:
    print("None")
else:
    print(ans[0])

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

[dic.remove(c) for c in charset]
if dic == []:
    print("None")
else:
    print(dic[0])

Solution (Python):
S = input()
d = {}

for i in range(ord("a"), ord("z") + 1):
    d[chr(i)] = False

for c in S:
    d[c] = True

ans = "None"
for i in range(ord("a"), ord("z") + 1):
    if not d[chr(i)]:
        ans = chr(i)
        break
print(ans)

Solution (Python):


n = input()
d=[]
abc =[chr(x) for x in range(97, 123)]
for i in abc:
    if i not in  n:
        d.append(i)
if d != []:
    print(sorted(d)[0])
else:
    print('None')

Solution (Python):
a = input()
b = ["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"]
i = 0
while len(a) > i:
    if a[i] in b:
        b.remove(a[i])
    else:
        pass
    i += 1
if not b:
    print("None")
else:
    print(b[0])

Solution (Python):
def main():
    s = set(list(input()))
    check = list("qwertyuiopasdfghjklzxcvbnm")
    check.sort()
    for i in check:
        if i not in s:
            print(i)
            return
    print("None")

main()

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

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


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

Solution (Python):

S = list(raw_input())

alpha = 'abcdefghijklmnopqrstuvwxyz'
t = True
for a in alpha:
    if a not in S:
        print a
        t = False
        break
if t:
    print None

Solution (Python):
S = input()
alf = "abcdefghijklmnopqrstuvwxyz"
li = []

for x in alf:
    if (x in S) == False:
      li.append(x)
      break

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

Solution (Python):
s = input()
ab_list = [True]*26
for i in s:
  ab_list[ord(i)-97] = False
for i in range(len(ab_list)):
  if ab_list[i]:
    print(chr(i+97))
    exit()
print("None")

Solution (Python):
S=input()

sorted(S)

if 'a' not in S:
    print('a')
elif 'b' not in S:
    print('b')
elif 'c' not in S:
    print('c')
elif 'd' not in S:
    print('d')
elif 'e' not in S:
    print('e')
elif 'f' not in S:
    print('f')
elif 'g' not in S:
    print('g')
elif 'h' not in S:
    print('h')
elif 'i' not in S:
    print('i')
elif 'j' not in S:
    print('j')
elif 'k' not in S:
    print('k')
elif 'l' not in S:
    print('l')
elif 'm' not in S:
    print('m')
elif 'n' not in S:
    print('n')
elif 'o' not in S:
    print('o')
elif 'p' not in S:
    print('p')
elif 'q' not in S:
    print('q')
elif 'r' not in S:
    print('r')
elif 's' not in S:
    print('s')
elif 't' not in S:
    print('t')
elif 'u' not in S:
    print('u')
elif 'v' not in S:
    print('v')
elif 'w' not in S:
    print('w')
elif 'x' not in S:
    print('x')
elif 'y' not in S:
    print('y')
elif 'z' not in S:
    print('z')
else:
    print('None')

Solution (Python):

s = str(input())
s = list(s)
x = 'abcdefghijklmnopqrstuvwxyz'
x = list(x)

flag = 0
for i in range(len(s)):
    try:
        if(len(x) == 1 and s[i] == x[0]):
            flag = 1
        x.remove(s[i])
    except:
        pass

if(flag == 1):
    print('None')
else:
    print(x[0])

Solution (Python):
hoge = [0] * 26
a = ord('a')
string = input()
for i in range(len(string)):
    hoge[ord(string[i])-a] = 1

for i in range(26):
    if hoge[i] == 0:
        print(chr(i+a))
        break
    if i == 25:
        print('None')

Solution (Python):
d = raw_input()

d_list = []
for i in range(len(d)):
    d_list.append(ord(d[i]))

ans = ord('a')
while True:
    if ans not in d_list:
        break
    if ans > ord('a')+122:
        break
    ans += 1

if ans > 122:
    print 'None'
else:
    print chr(ans)