Problem:
 Score : 200 points    Problem Statement You are given a string S consisting of lowercase English letters.
Find the lexicographically (alphabetically) smallest lowercase English letter that does not occur in S .
If every lowercase English letter occurs in S , print None instead.      Constraints  1 \leq |S| \leq 10^5 ( |S| is the length of string S .)  S consists of lowercase English letters.         Input Input is given from Standard Input in the following format:  S       Output Print the lexicographically smallest lowercase English letter that does not occur in S .
If every lowercase English letter occurs in S , print None instead.        Sample Input 1 atcoderregularcontest      Sample Output 1 b  The string atcoderregularcontest contains a , but does not contain b .       Sample Input 2 abcdefghijklmnopqrstuvwxyz      Sample Output 2 None  This string contains every lowercase English letter.       Sample Input 3 fajsonlslfepbjtsaayxbymeskptcumtwrmkkinjxnnucagfrg      Sample Output 3 d

Solution (Python):

s = input()

s_set = set(s)
alphabet = "abcdefghijklmnopqrstuvwxyz"
flg = False
for i in range(len(alphabet)):
    if not alphabet[i] in s_set:
        print(alphabet[i])
        flg = True
        break
if not flg:
    print(None)

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

Solution (Python):

























s = input()
chars = set('abcdefghijklmnopqrstuvxwyz')
x = list(sorted(chars - set(s)))
if len(x):
    print(x[0])
else:
    print('None')

Solution (Python):
s=input()
x=ord('a')
z=0
for i in range(x,x+26):
    y=chr(i)
    if y not in s:
        print(y)
        z=1
        break
if z==0:
    print('None')

Solution (Python):
s = input()
chars = set('abcdefghijklmnopqrstuvxwyz')
x = list(sorted(chars - set(s)))
if len(x):
    print(x[0])
else:
    print('None')

Solution (Python):
def main():
    S = input()
    A = [False]*26
    a = ord("a")
    for char in S:
        A[ord(char) - a] = True
    for i in range(26):
        if not A[i]:
            print(chr(i+a))
            return
    print("None")

main()

Solution (Python):

import sys
from collections import defaultdict, deque




def solve():
    s = set([x for x in input()])
    for i in range(26):
        e = chr(ord('a') + i)
        if e not in s:
            print(e)
            return
    print('None')
        
t = 1

for case in range(1,t+1):
    ans = solve()



"""

1 4
7
RRRR
1 1 0 1
----
4

1 4
47
RCRR
1 1 0 1
----
-1

2 2
10
RR
RR
1 7
4 1
----
4

2 2
10
RR
RR
7 4
7 4
----
-1

3 3
8
RCR
RRC
RRR
1 1 1
1 1 1
1 3 1


3 3
1
RCR
RRC
RRR
1 1 1
1 1 1
1 3 1


"""

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

a = ord("a")
z = ord("z")
X = []
for i in range(len(S)):
  X.append(ord(S[i]))

for j in range(a,z+1):
  if j not in X:
    print(chr(j))
    exit()
    
print("None")

Solution (Python):
S=input()
L=("a","b","c","d","e","f","g","h","i","j","k","l","m","n",
    "o","p","q","r","s","t","u","v","w","x","y","z")
for l in L:
    if S.count(l)>0:
        pass
    else:
        print(l)
        exit()
else:
    print("None")

Solution (Python):
import math
from collections import deque
from collections import defaultdict

import bisect


def readInt():
	return int(input())
def readInts():
	return list(map(int, input().split()))
def readChar():
	return input()
def readChars():
	return input().split()
def factorization(n):
	res = []
	if n%2==0:
		res.append(2)
	for i in range(3,math.floor(n//2)+1,2):
		if n%i==0:
			c = 0
			for j in res:
				if i%j==0:
					c=1
			if c==0:
				res.append(i)
	return res
def fact2(n):
	p = factorization(n)
	res = []
	for i in p:
		c=0
		z=n
		while 1:
			if z%i==0:
				c+=1
				z/=i
			else:
				break
		res.append([i,c])
	return res
def fact(n):
	ans = 1
	m=n
	for _i in range(n-1):
		ans*=m
		m-=1
	return ans
def comb(n,r):
	if n<r:
		return 0
	l = min(r,n-r)
	m=n
	u=1
	for _i in range(l):
		u*=m
		m-=1
	return u//fact(l)
def combmod(n,r,mod):
	return (fact(n)/fact(n-r)*pow(fact(r),mod-2,mod))%mod
def printQueue(q):
	r=copyQueue(q)
	ans=[0]*r.qsize()
	for i in range(r.qsize()-1,-1,-1):
		ans[i] = r.get()
	print(ans)
class UnionFind():
	def __init__(self, n):
		self.n = n
		self.parents = [-1]*n

	def find(self, x): 
		if self.parents[x]<0:
			return x
		else:
			self.parents[x] = self.find(self.parents[x])
			return self.parents[x]

	def union(self,x,y):
		x = self.find(x)
		y = self.find(y)

		if x==y:
			return

		if self.parents[x]>self.parents[y]:
			x,y = y,x

		self.parents[x]+=self.parents[y]
		self.parents[y]=x

	def size(self,x):
		return -1*self.parents[self.find(x)]

	def same(self,x,y):
		return self.find(x)==self.find(y)

	def members(self,x): 
		root = self.find(x)
		return [i for i in range(self.n) if self.find(i)==root]

	def roots(self):
		return [i for i,x in enumerate(self.parents) if x<0]

	def group_count(self):
		return len(self.roots())

	def all_group_members(self):
		return {r: self.members(r) for r in self.roots()} 
def bitArr(n):
	x = 1
	zero = "0"*n
	ans = []
	ans.append([0]*n)
	for i in range(2**n-1):
		ans.append(list(map(lambda x:int(x),list((zero+bin(x)[2:])[-1*n:]))))
		x+=1
	return ans;
def arrsSum(a1,a2):
	for i in range(len(a1)):
		a1[i]+=a2[i]
	return a1
def maxValue(a,b,v):
	v2 = v
	for i in range(v2,-1,-1):
		for j in range(v2//a+1): 
			k = i-a*j
			if k%b==0:
				return i
	return -1
def copyQueue(q):
	nq = queue.Queue()
	n = q.qsize()
	for i in range(n):
		x = q.get()
		q.put(x)
		nq.put(x)
	return nq
def get_sieve_of_eratosthenes(n):
	
	data = [0,0,0]
	for i in range(3,n+1,2):
		data.append(i)
		data.append(0)
	for i in range(len(data)):
		interval = data[i]
		if interval!=0:
			for j in range(i+interval,n-1,interval):
				data[j] = 0
	
	ans = data[:]
	return ans

def isInt(n):
	if isinstance(n, int):
		return True
	else:
		return n.is_integer()

s = readChar()

ex = set()

for i in s:
	ex.add(i)

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(len(ex)):
	al.remove(ex.pop())

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

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

b = Counter(s)

z = [0] * 26
for items,values in b.items():
    z[ord(items)- 97] = 1

for i in range(26):
    if z[i] == 0:
        print(chr(i+97))
        exit(0)
print("None")

Solution (Python):
def counting_sort(array,max):
    bucket = [0] * (max+1)

    for i in range(len(array)):
        if array[i] == 'a':
            bucket[0] += 1
        elif array[i] == 'b':
            bucket[1] += 1
        elif array[i] == 'c':
            bucket[2] += 1
        elif array[i] == 'd':
            bucket[3] += 1
        elif array[i] == 'e':
            bucket[4] += 1
        elif array[i] == 'f':
            bucket[5] += 1
        elif array[i] == 'g':
            bucket[6] += 1
        elif array[i] == 'h':
            bucket[7] += 1
        elif array[i] == 'i':
            bucket[8] += 1
        elif array[i] == 'j':
            bucket[9] += 1
        elif array[i] == 'k':
            bucket[10] += 1
        elif array[i] == 'l':
            bucket[11] += 1
        elif array[i] == 'm':
            bucket[12] += 1
        elif array[i] == 'n':
            bucket[13] += 1
        elif array[i] == 'o':
            bucket[14] += 1
        elif array[i] == 'p':
            bucket[15] += 1
        elif array[i] == 'q':
            bucket[16] += 1
        elif array[i] == 'r':
            bucket[17] += 1
        elif array[i] == 's':
            bucket[18] += 1
        elif array[i] == 't':
            bucket[19] += 1
        elif array[i] == 'u':
            bucket[20] += 1
        elif array[i] == 'v':
            bucket[21] += 1
        elif array[i] == 'w':
            bucket[22] += 1
        elif array[i] == 'x':
            bucket[23] += 1
        elif array[i] == 'y':
            bucket[24] += 1
        elif array[i] == 'z':
            bucket[25] += 1
    return (bucket)

def main():
    S = input()
    bucket_list = counting_sort(S, 25)
    for i in range(26):
        if bucket_list[i] == 0:
            if i == 0:
                print('a')
                return
            if i == 1:
                print('b')
                return
            if i == 2:
                print('c')
                return
            if i == 3:
                print('d')
                return
            if i == 4:
                print('e')
                return
            if i == 5:
                print('f')
                return
            if i == 6:
                print('g')
                return
            if i == 7:
                print('h')
                return
            if i == 8:
                print('i')
                return
            if i == 9:
                print('j')
                return
            if i == 10:
                print('k')
                return
            if i == 11:
                print('l')
                return
            if i == 12:
                print('m')
                return
            if i == 13:
                print('n')
                return
            if i == 14:
                print('o')
                return
            if i == 15:
                print('p')
                return
            if i == 16:
                print('q')
                return
            if i == 17:
                print('r')
                return
            if i == 18:
                print('s')
                return
            if i == 19:
                print('t')
                return
            if i == 20:
                print('u')
                return
            if i == 21:
                print('v')
                return
            if i == 22:
                print('w')
                return
            if i == 23:
                print('x')
                return
            if i == 24:
                print('y')
                return
            if i == 25:
                print('z')
                return
    print('None')

main()

Solution (Python):
li=list("abcdefghijklmnopqrstuvwxyz")
inp=list(input())
used=[]
for c in inp:
  if c in used:
    continue
  li.remove(c)
  used.append(c)

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

Solution (Python):
S = input()
ans = "None"
W = {"a":0, "b":0, "c":0, "d":0,"e":0,"f":0,"g":0,"h":0,"i":0,"j":0,"k":0,"l":0,"m":0,"n":0,"o":0,"p":0,"q":0,"r":0,"s":0,"t":0,"u":0,"v":0,"w":0,"x":0,"y":0,"z":0}
P = "abcdefghijklmnopqrstuvwxyz"
for i in S:
  W[i] += 1
for p in P:
  if W[p] == 0:
    ans = p
    break
print(ans)

Solution (Python):



import sys



def II(): return int(sys.stdin.readline())
def MI(): return map(int, sys.stdin.readline().split())
def LI(): return list(map(int, sys.stdin.readline().split()))
def LLI(rows_number): return [LI() for _ in range(rows_number)]


def solver(inputString):
    full_set = set()
    input_set = set()
    for j in range(ord('a'), ord('z')+1, +1):
        full_set.add(chr(j))
    for j in range(0, len(inputString), +1):
        input_set.add(inputString[j])
    
    
    
    full_set -= input_set
    
    fullset_list = list(full_set)
    fullset_list.sort()
    
    if 0 == len(fullset_list):
        result = "None"
    else:
        result = fullset_list[0]
    return result


if __name__ == "__main__":
    S = sys.stdin.readline().split()
    print("{}".format(solver(S[0])))

Solution (Python):
s = str(input())
s = sorted(set(s))
ch = "abcdefghijklnopqrstuvwxyz"
for i in ch:
    if i not in s:
        print(i)
        break
else:
    print("None")

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

Solution (Python):
s=input()
al="abcdefghijklmnopqrstuvwxyz"
ans="None"
for i in al:
  dumy=s.count(i)
  if dumy==0:
    ans=i
    break

print(ans)

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

for j in range(26):
  if l[j] in s:
    j += 1
  else:
    print(l[j])
    exit()
print('None')

Solution (Python):
def solve():
    S = set(input())
    alph = set("qwertyuiopasdfghjklzxcvbnm")
    nonS = sorted(alph-S)
    if len(nonS) == 0:
        print("None")
    else:
        print(nonS[0])


if __name__ == '__main__':
    solve()

Solution (Python):
s = input()
c = set()
for i in range(len(s)):
  c.add(s[i])
c = list(c)
c.sort()
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']
ans = ''
for e in abc:
  if e not in c:
    ans = e
    break
if ans == '':
  ans = 'None'
print(ans)

Solution (Python):
s = input()

l = [chr(i) for i in range(97, 97 + 26)]
flag = True
for i in l:
    if not i in s:
        print(i)
        flag = False
        break
if flag:
    print('None')

Solution (Python):
s = input()

l = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z']
flag = True
for i in l:
    if not i in s:
        print(i)
        flag = False
        break
if flag:
    print('None')

Solution (Python):
S = list(input())
alphabet = ['a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z']


print('None') if len(set(S)) == 26 else print(sorted(list(set(alphabet) - set(S)))[0])

Solution (Python):
s=input()
alphabet=[chr(i) for i in range(97,97+26)]
for i in alphabet:
    if s.count(i)<1:
        print(i)
        exit()
print("None")

Solution (Python):
import string

s = set(input())
x = set(string.ascii_lowercase)

print('None' if x == s else sorted(x - s)[0])

Solution (Python):
S = input()

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

remain = list(set(alpha) - set(S))

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

Solution (Python):
s=input()
sset=set(s)
list1=list(range(97,123))
for i in range(len(sset)):
    str1=sset.pop()
    list1.remove(ord(str1))
if len(list1)==0:
    print("None")
else:
    print(chr(list1[0]))

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

Solution (Python):
S = input()
alpha = "abcdefghijklmnopqrstuvwxyz"
D = {alpha[i]: [i, 0] for i in range(len(alpha))}
for i in S:
  D[i][1] += 1
L = [D[i] for i in D]
L.sort()
if min(L[i][1] for i in range(len(alpha))) >= 1:
  print("None")
else:
  for i in range(len(alpha)):
    if L[i][1] == 0:
      break
  print(alpha[i])

Solution (Python):


import sys





import string                                
























def main():
    mod = 1000000007                
    inf = float('inf')              
    
    sys.setrecursionlimit(10**6)    
    def input(): return sys.stdin.readline().rstrip()
    def ii():    return int(input())
    def mi():    return map(int, input().split())
    def mi_0():  return map(lambda x: int(x)-1, input().split())
    def lmi():   return list(map(int, input().split()))
    def lmi_0(): return list(map(lambda x: int(x)-1, input().split()))
    def li():    return list(input())
    
    
    s = input()
    for alpha in string.ascii_lowercase:
        if alpha in s:
            continue
        else:
            print(alpha)
            exit()
    print('None')


if __name__ == "__main__":
    main()

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

Solution (Python):
from string import ascii_lowercase

S = input()
D = dict()

for s in S:
    D[s] = True

for s in ascii_lowercase:
    if not s in D:
        print(s)
        exit()
print('None')

Solution (Python):

if __name__ == '__main__':
    a = input()
    count = 0

    for i in range(26):
        s = chr(97+i)
        if s in a:
            count+=1
        else:
            print(s)
            break

    if count == 26:
        print("None")

Solution (Python):
s = list(input())
s2 = []
for i in range(len(s)):
    s2.append(s[i])
s3 = list(set(s2))
s3.sort()
s4 = ['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 s3 == s4:
    print('None')
else:
    for i in range(int(len(s3))):
        if s3[i] != s4[i]:
            print(s4[i])
            break
        if i == len(s3)-1:
            print(s4[i+1])

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

Solution (Python):
S = input()
S = sorted(set(S))
alf =sorted(set("abcdefghijklmnopqrstuvwxyz"))
ans = ""

if len(S) == 26:
    print("None")
else:
    for i in range(len(alf)):
        if alf[i] not in S:
            ans += alf[i]
    ans = sorted(ans)  
    print(ans[0])

Solution (Python):
S=sorted(set(list(str(input()))))
alf='abcdefghijklmnopqrstuvwxyz'
if len(S)==26:
  print('None')
else:
  for i in range(26):
    if alf[i] not in S:
      print(alf[i])
      break

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

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

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

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

Solution (Python):
import sys

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

for i in range(0,len(S)):
  tmp.append(ord(S[i]))

for i in range(97,123):
  if (i in tmp):
    print("", end="")
  else:
    print (chr(i))
    sys.exit()
print("None")

Solution (Python):

import sys


def solve(S: str):
    alp = sorted(set(S))
    for abc in [chr(i) for i in range(97, 97 + 26)]:
        if not abc in alp:
            print(abc)
            break
    else:
        print("None")
    return



def main():
    def iterate_tokens():
        for line in sys.stdin:
            for word in line.split():
                yield word

    tokens = iterate_tokens()
    S = next(tokens)  
    solve(S)


if __name__ == "__main__":
    main()

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

Solution (Python):








S= str(input())

dict = list('abcdefghijklmnopqrstuvwxyz')
for i in range(len(S)):
    for s in dict:
        if s==S[i]:
            dict.remove(s)

            continue
if dict:
    print(dict[0])

else:
    print('None')

Solution (Python):
import string

word = input()
word = set(word)

chars = set(string.ascii_lowercase)
diff = chars - word
sorted_diff = sorted(diff)
if sorted_diff:
    print(sorted_diff[0])
else:
    print('None')

Solution (Python):
s = sorted({chr(q) for q in range(97, 123)} - set(input()))
print(s[0] if s else "None")

Solution (Python):
def main():
    s = list(input())
    s_d = list(set(s))
    a = [chr(i) for i in range(ord('a'), ord('z')+1)]
    set_diff = sorted(list(set(s_d) ^ set(a)))
    if len(set_diff) > 0:
        print(set_diff[0])
    else:
        print('None')
if __name__ == '__main__':
    main()

Solution (Python):



import sys



def II(): return int(sys.stdin.readline())
def MI(): return map(int, sys.stdin.readline().split())
def LI(): return list(map(int, sys.stdin.readline().split()))
def LLI(rows_number): return [LI() for _ in range(rows_number)]
def solver(inputStr):
    
    
    bucket = [0] * 26
    result = "None"
    
    for j in range(0, len(inputStr), +1):
        charNo = ord(inputStr[j]) - ord('a')
        bucket[charNo] = bucket[charNo] + 1
    for j in range(0, 26, +1):
        if bucket[j] == 0:
            result = chr(ord('a')+j)
            break
    
    return result
if __name__ == "__main__":
    S = sys.stdin.readline().split()
    print("{}".format(solver(S[0])))

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 = ["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 set(S):
	A.remove(i)
	
if len(A) == 0:
	print("None")
else:
	print(A[0])

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

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

print('None')

Solution (Python):
import string
S = input()
v = set()
for c in S:
    v.add(c)

for c in string.ascii_lowercase:
    if not c in v:
        print(c)
        break
else:
    print("None")

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

import collections

c = collections.Counter(sls)
temp = "abcdefghijklmnopqrstuvwxyz"

for i in temp:
    if c[i]==0:
        print(i)
        exit()
print("None")

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

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

Solution (Python):
import string
def ABC_71_B():
    s=list(input())
    li_all = [x for x in string.ascii_letters]

    if len(set(s))>=26:
        print('None')
    else:
        s.sort()
        for i in range(len(s)):
            if s.count(li_all[i])==0:
                print(li_all[i])
                break
    
if __name__ == '__main__':

    ABC_71_B()

Solution (Python):
s=list(input())
flag=0
s=sorted(set(s))
list1='abcdefghijklmnopqrstuvwxyz'
for i in range(26):
    if list1[i] not in s:
        flag=1
        print(list1[i])
        break
if flag==0:
    print('None')

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

Solution (Python):
S = set(input())
if len(S)==26:
    print('None')
else:
    A = 'abcdefghijklmnopqrstuvwxyz'
    for i in range(26):
        if A[i] not in S:
            print(A[i])
            break

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

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

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

keys = list(dict.keys())
for i in range(ord('a'), ord('z')+1):
    if chr(i) in keys:
        continue
    else:
        print(chr(i))
        exit()
print(None)

Solution (Python):
import collections

def main():
    S = list(input())
    CS = collections.Counter(S)
    letter = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z']
    for l in letter:
        if l not in CS:
            print(l)
            return
    print('None')

if __name__ == "__main__":
    main()

Solution (Python):
def solve():
    ans = 0
    S = input()
    lis = [0]*26
    for s in S:
        lis[ord(s)-ord('a')] += 1
    for i,l in enumerate(lis):
        if l==0:
            ans = chr(ord('a')+i)
            return ans
    return 'None'
print(solve())

Solution (Python):
Ss=set(input())
S=sorted(Ss)
if len(S)==26:
    print('None')
    exit()
else:
    for i in range(26):
        if not chr(i+97) in S:
            print(chr(i+97))
            exit()

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

Solution (Python):
import sys

import math


from collections import deque

from fractions  import gcd


s=input()
n=len(s)
al=[chr(ord('a') + i) for i in range(26)]
ind=[0]*26
for i in range(n):
    j=ord(s[i])-ord("a")
    ind[j]-=1
for i in range(26):
    if ind[i]==0:
        print(al[i])
        break
else:
    print("None")

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

alpha = "abcdefghijklmnopqrstuvwxyz"

for i in range(N):
    alpha = alpha.replace(S[i], "")

ans = sorted(alpha)

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

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

Solution (Python):
from string import ascii_lowercase


S = input()
ans = None
for c in ascii_lowercase:
    if c not in S:
        ans = c
        break
print(ans)

Solution (Python):
from string import ascii_lowercase


S = set(input())
for c in 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):
  c = chr(i)
  if s.count(c) == 0:
    print(c)
    break
else:
  print('None')

Solution (Python):
import string
s = set(list(input()))
e = list(string.ascii_lowercase)

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

print("None")

Solution (Python):



import sys



def II(): return int(sys.stdin.readline())
def MI(): return map(int, sys.stdin.readline().split())
def LI(): return list(map(int, sys.stdin.readline().split()))
def LLI(rows_number): return [LI() for _ in range(rows_number)]

def initSet():
    result = set()
    for j in range(ord('a'),ord('z')+1):
        result.add(chr(j))
    return result

def solver(inputString,allCharSet):
    result = ''
    inputSet = set(inputString)
    
    
    diffset = allCharSet - inputSet
    
    difflist = list(diffset)
    difflist.sort()
    
    if len(difflist) == 0:
        result = "None"
    else:
        result = difflist[0]
    return result

if __name__ == "__main__":
    S = sys.stdin.readline().split('\n')
    allset = initSet()
    print("{}".format(solver(S[0],allset)))

Solution (Python):
S = input()
alp = [chr(i) for i in range(97, 97 + 26)]
for c in alp:
    if c in S:
        pass
    else:
        print(c)
        break
else:
    print('None')

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

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

if len(s) == 26:
    print("None")
else:
    print(sorted("".join(set("abcdefghijklmnopqrstuvwxyz") - s))[0])

Solution (Python):
list=[chr(i) for i in range(97, 97+26)]
S=input()
S=set(S)
A=[x for x in S]
A=sorted(A)
if len(A)<26:
    A+="aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
for i in range(26):
    if A[i] == list[i]:
        pass
    else:
        print(list[i])
        exit()
print("None")

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

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

Solution (Python):
s=input()
for i in range(26):
  if s.count(chr(ord('a')+i))==0:
    print(chr(ord('a')+i))
    break
else:
  print('None')

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

Solution (Python):
S=input()
alphabet=[chr(i) for i in range(97,97+26)]
for i in alphabet:
    if i in S:
        continue
    else:
        print(i)
        exit()
print("None")

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

Solution (Python):
S = input()

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

for al in S:
    s_list.append(al)

if set(s_list) == set(al_list):
    print('None')
else:
    print(min(list(set(al_list) - set(s_list))))

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','None']

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

print(alpha[0])

Solution (Python):
S = input()

for i in range(97,26+97):
    if chr(i) not in S:
        print(chr(i))
        exit(0)
else:
    print('None')

Solution (Python):
s = input()

for i in range(97,97+26):
    if chr(i) not in s:
        print(chr(i))
        exit()
print("None")

Solution (Python):

s = sorted(map(ord, set(input())))
alp = ord('a')

for c in s:
    if c == alp:
        if c == ord('z'):
            print('None')
            quit()
        else:
            alp += 1
            continue
    else:
        print(chr(alp))
        quit()
print(chr(alp))

Solution (Python):
s = input()
t = [n for n in range(97,97+26)]
for ss in s:
	tmp = ord(ss)
	if tmp in t:
		t.remove(tmp)
if len(t) > 0:
	print(chr(t[0]))
else:
	print("None")

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

s = list(s)
a = list(a)

for i in range(0,len(s)):
  if(s[i] in a):
    k = a.index(s[i])
    del a[k]

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

Solution (Python):
s = input()
ll = [i for i in range(97,123,1)]
for j in s:
    s_num = ord(j)
    if s_num in ll:
        ll.remove(s_num)
    else:
        pass
if len(ll) != 0:
    print(chr(ll[0]))
else:
    print("None")

Solution (Python):
import sys, re
from math import ceil, sqrt, hypot, factorial, pi, sin, cos, tan, asin, acos, atan, radians, degrees, log2
from collections import deque, defaultdict, Counter
from itertools import accumulate, permutations, combinations, combinations_with_replacement, product, groupby
from operator import itemgetter, mul
from copy import deepcopy
from string import ascii_lowercase, ascii_uppercase, digits
from bisect import bisect, bisect_left
from fractions import gcd
from heapq import heappush, heappop, heapify
from functools import reduce
def input(): return sys.stdin.readline().strip()
def INT(): return int(input())
def MAP(): return map(int, input().split())
def LIST(): return list(map(int, input().split()))
def ZIP(n): return zip(*(MAP() for _ in range(n)))
sys.setrecursionlimit(10 ** 9)
INF = float('inf')
mod = 10 ** 9 + 7

S = input()

ans = list(ascii_lowercase)

for x in S:
	if x in ans:
		ans.remove(x)

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

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

Solution (Python):
S=list(input())
l=sorted(list(set(S)))
for (i,j) in zip(range(len(l)),range(97,97+26)):
    if chr(j) !=l[i]:
        print(chr(j))
        exit()
    elif chr(j) == 'z':
        print('None')
        exit()
print(chr(j+1))

Solution (Python):
s = input()

for c in range(97,123):
  if not(chr(c) in s):
    print(chr(c))
    break
else:
  print("None")

Solution (Python):
S = sorted(set(map(ord, input())))
for c in range(ord('a'), ord('z') + 1):
    if c in S:
        continue
    print(chr(c))
    break
else:
    print('None')

Solution (Python):
def instr():
    s = input()
    return(list(s[:len(s)]))


c = "abcdefghijklmnopqrstuvwxyz"
s = instr()

res = "None"
for i in c:
    if i not in s:
        res = i
        break

print(res)