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-----
atcoderregularcontest

-----Sample Output-----
b

The string atcoderregularcontest contains a, but does not contain b.

Difficulty: EASY

Starter code:

Solutions:
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']
for a in alphabet:
	if a not in S:
		print(a)
		return
print('None')


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


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


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


import sys
import copy
import math
import itertools
import numpy as np
S = input()
for i in 'abcdefghijklmnopqrstuvwxyz':
	if S.count(i) == 0:
		print(i)
		return
print('None')


S = input()
l = sorted(list(set(S)))
ans = ''
for i in range(len(l)):
	if ord(l[i]) != 97 + i:
		ans = chr(97 + i)
		break
	elif i == 25 and l[i] == 'z':
		ans = 'None'
if ans != '':
	print(ans)
else:
	print(chr(ord(l[-1]) + 1))


S = str(input())
s = 'abcdefghijklmnopqrstuvwxyz'
for i in s:
	if i not in S:
		print(i)
		return
print('None')


import sys
s = input()
for i in range(97, 123):
	st = chr(i)
	if st not in s:
		print(st)
		return
print(None)


import string
s = list(map(str, input()))
s = list(set(s))
s.sort()
ans = ''
arr = []
alp = list(map(str, string.ascii_lowercase))
for char in s:
	alp.remove(char)
if len(alp) == 0:
	print('None')
else:
	print(alp[0])


import string
s = input()
t = set(s) ^ set(string.ascii_lowercase)
for i in string.ascii_lowercase:
	if set(i) & t:
		print(i)
		break
else:
	print('None')


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


s = list(input())
s.sort()
li = []
lis = [97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122]
for i in s:
	li.append(ord(i))
cnt = 0
for i in lis:
	if i in li:
		cnt += 1
	else:
		print(chr(i))
		return
print('None')


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


s = input()
alphabets = 'abcdefghijklmnopqrstuvwxyz'
for i in range(len(alphabets)):
	if not alphabets[i] in s:
		print(alphabets[i])
		return
print('None')


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


import sys
import math
import itertools
import collections
import heapq
import re
import numpy as np
from functools import reduce
rr = lambda : sys.stdin.readline().rstrip()
rs = lambda : sys.stdin.readline().split()
ri = lambda : int(sys.stdin.readline())
rm = lambda : list(map(int, sys.stdin.readline().split()))
rl = lambda : list(map(int, sys.stdin.readline().split()))
inf = float('inf')
mod = 10 ** 9 + 7
s = rr()
for char in 'abcdefghijklmnopqrstuvwxyz':
	if char not in s:
		print(char)
		return
else:
	print('None')


import math
from math import gcd, pi, sqrt
INF = float('inf')
import sys
sys.setrecursionlimit(10 ** 6)
import itertools
from collections import Counter, deque

def i_input():
	return int(input())

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

def i_list():
	return list(i_map())

def i_row(N):
	return [i_input() for _ in range(N)]

def i_row_list(N):
	return [i_list() for _ in range(N)]

def s_input():
	return input()

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

def s_list():
	return list(s_map())

def s_row(N):
	return [s_input for _ in range(N)]

def s_row_str(N):
	return [s_list() for _ in range(N)]

def s_row_list(N):
	return [list(s_input()) for _ in range(N)]
import string

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

def __starting_point():
	main()
__starting_point()


s = input()
for i in range(ord('a'), ord('z') + 1):
	if chr(i) not in s:
		print(chr(i))
		return
print('None')


from collections import Counter
S = list(input())
ac = Counter(S)
alp = 'abcdefghijklmnopqrstuvwxyz'
for a in alp:
	if not a in ac:
		print(a)
		break
else:
	print('None')


import string
S = str(input())
set1 = set()
for s in S:
	set1.add(s)
set2 = set()
for alf in string.ascii_lowercase:
	set2.add(alf)
list = sorted(set2 - set1)
if not list:
	print('None')
else:
	print(sorted(list)[0])


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']
ans = []
for alpha in alphabets:
	if S.count(alpha) > 0:
		ans.append(1)
	else:
		ans.append(0)
if sum(ans) == 26:
	print('None')
else:
	print(alphabets[ans.index(0)])


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


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


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


li = list('abcdefghijknmlopqrstuvwxyz')
S = list(input())
ans = 'None'
for l in li:
	if not l in S:
		ans = l
		break
print(ans)


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


s = set(input())
a = 'abcdefghijklmnopqrstuvwxyz'
for si in s:
	a = a.replace(si, '')
print(None if a == '' else a[0])


s = list(input())
import collections
import sys
a = collections.Counter(s)
for i in range(97, 97 + 26):
	if chr(i) not in a:
		print(chr(i))
		return
else:
	print('None')


S = input()
import string
lst = list(string.ascii_lowercase)
d = {lst[i]: i for i in range(len(lst))}
cnt = [0] * len(lst)
for ch in S:
	cnt[d[ch]] = 1
idx = -1
for i in range(len(cnt)):
	if cnt[i] == 0:
		idx = i
		break
if idx == -1:
	print('None')
else:
	print(lst[idx])


d = {}
for s in input():
	if s in d:
		d[s] += 1
	else:
		d[s] = 1
for a in 'abcdefghijklmnopqrstuvwxyz':
	if a not in d:
		print(a)
		return
print('None')


l = set('abcdefghijklmnopqrstuvwxyz')
s = set(input())
ans = sorted(list(l ^ s))
print('None' if len(ans) == 0 else ans[0])


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


S = input()
alph = 'abcdefghijklmnopqrstuvwxyz'
for i in alph:
	if i in S:
		alph = alph.replace(i, '')
if len(alph) == 0:
	print('None')
else:
	print(alph[0])


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


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


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


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


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


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


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


def main():
	s = input()
	for x in range(ord('a'), ord('z') + 1):
		c = chr(x)
		if c in s:
			continue
		else:
			print(c)
			return
	print('None')
main()


S = sorted(list(set(input())))
alphabet = list('abcdefghijklmnopqrstuvwxyz')
for i in range(len(alphabet)):
	if alphabet[i] not in S:
		print(alphabet[i])
		return
print('None')


S = input()
a = [0] * 26
for i in range(len(S)):
	tmp = ord(S[i]) - ord('a')
	a[tmp] = 1
for i in range(26):
	if a[i] == 0:
		tmp = chr(ord('a') + i)
		print(tmp)
		break
else:
	print('None')


S = list(input())
S.sort()
if S[0] != 'a':
	print('a')
elif S[0] == 'a':
	for i in range(len(S) - 1):
		if ord(S[i + 1]) - ord(S[i]) == 0 or ord(S[i + 1]) - ord(S[i]) == 1:
			pass
		else:
			res = ord(S[i]) + 1
			print(chr(res))
			return
	res = ord(S[-1]) + 1
	if S[0] == 'a' and S[-1] == 'z':
		print('None')
	elif S[0] != 'a':
		print('a')
	else:
		print(chr(res))


S = set(input())
az = [chr(i) for i in range(ord('a'), ord('z') + 1)]
if len(S) >= 26:
	print('None')
	return
for i in az:
	if i in S:
		pass
	else:
		print(i)
		return


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


S = sorted(set(list(input())))
ans = 'None'
for keyword in list('abcdefghijklmnopqrstuvwxyz'):
	hit = False
	for s in S:
		if s == keyword:
			hit = True
			break
	if hit == False:
		ans = keyword
		break
print(ans)


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


from fractions import gcd
from itertools import combinations, permutations, accumulate, product
from collections import deque, defaultdict, Counter
import decimal
import re
import math
import bisect
import sys
sys.setrecursionlimit(10000000)
mod = 10 ** 9 + 7
from sys import stdin
readline = stdin.readline

def readInts():
	return list(map(int, readline().split()))

def readTuples():
	return tuple(map(int, readline().split()))

def I():
	return int(readline())
ans = 'abcdefghijklmnopqrstuvwxyz'
s = input()
s = sorted(list(set(s)))
for v in ans:
	flag = True
	for j in range(len(s)):
		if s[j] == v:
			flag = False
	if flag:
		print(v)
		return
print(None)


S = input()
abc = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z']
from collections import defaultdict
d = defaultdict(int)
for s in S:
	d[s] = 1
for a in abc:
	if d[a] == 0:
		print(a)
		return
print('None')


s = input()
alphabets = [0] * 26
for i in s:
	alphabets[ord(i) - ord('a')] += 1
for i in range(26):
	if alphabets[i] == 0:
		print(chr(i + ord('a')))
		break
else:
	print('None')


s = list(input())
flag = [0] * 26
for i in range(len(s)):
	flag[ord(s[i]) - 97] = 1
c = 0
for i in range(26):
	if flag[i] == 0:
		print(chr(i + 97))
		break
	else:
		c += 1
if c == 26:
	print('None')


S = input()
N = len(S)
for i in range(26):
	for k in range(N):
		if S[k] == chr(97 + i):
			break
	else:
		print(chr(97 + i))
		return
print('None')


alpha = 'abcdefghijklmnopqrstuvwxyz'
S = input()
for i in range(len(alpha)):
	if alpha[i] not in S:
		print(alpha[i])
		return
print('None')


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


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


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


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


import sys
input = sys.stdin.readline

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

def __starting_point():
	main()
__starting_point()


S = str(input())
no_ans = 'None'
ans_list = []
arufabet = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z']
for i in range(len(arufabet)):
	if arufabet[i] not in S:
		ans_list.append(arufabet[i])
if len(ans_list) == 0:
	print(no_ans)
else:
	print(ans_list[0])


S = [s for s in input()]
li = list(set(S))
li.sort()
ans = 'None'
for c in [chr(c) for c in range(97, 123)]:
	if c not in li:
		ans = c
		break
print(ans)


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


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


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


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


S = input()
a = [True for _ in range(26)]
for s in S:
	a[ord(s) - ord('a')] = False
for (i, aa) in enumerate(a):
	if aa:
		print(chr(i + ord('a')))
		return
print('None')


s = input()
alphabet = 'abcdefghijklmnopqrstuvwxyz'
for i in alphabet:
	if i not in s:
		print(i)
		return
print('None')


S = list(input())
a = list('abcdefghijklmnopqrstuvwxyz')
for i in S:
	if i in a:
		a.remove(i)
if a:
	print(a[0])
else:
	print('None')


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


from math import gcd
from itertools import combinations, permutations, accumulate, product
from collections import deque, defaultdict, Counter
import decimal
import re
import math
import bisect
import heapq
import sys
sys.setrecursionlimit(10000000)
mod = 10 ** 9 + 7
INF = float('inf')
from sys import stdin
readline = stdin.readline

def readInts():
	return list(map(int, readline().split()))

def readTuples():
	return tuple(map(int, readline().split()))

def I():
	return int(readline())
s = input()
lis = 'abcdefghijklmnopqrstuvwxyz'
for i in range(len(lis)):
	if lis[i] in s:
		pass
	else:
		print(lis[i])
		return
print('None')


S = input()
T = 'abcdefghijklmnopqrstuvwxyz'
for c in T:
	if S.count(c) == 0:
		print(c)
		break
else:
	print('None')


S = set(list(input()))
cf = [chr(ord('a') + i) for i in range(26)]
for i in range(len(cf)):
	if cf[i] not in S:
		print(cf[i])
		return
print('None')


import string
all_string = set(string.ascii_lowercase)
S = set(input())
ans = sorted(list(all_string - S))
print(ans[0]) if ans != [] else print('None')


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


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


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


alphabets = [chr(ord('a') + i) for i in range(26)]
for a in list(input()):
	if a in alphabets:
		alphabets.remove(a)
if alphabets:
	print(alphabets[0])
else:
	print('None')


from string import ascii_lowercase

def answer(s: str) -> str:
	not_in_s = list(set(ascii_lowercase) - set(s))
	if not not_in_s:
		return 'None'
	not_in_s.sort()
	return not_in_s[0]

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

def __starting_point():
	main()
__starting_point()


import sys
alphabet = 'abcdefghijklmnopqrstuvwxyz'
s = set(sorted(input().strip()))
for i in alphabet:
	if not i in s:
		print(i)
		return
print('None')


s = str(input())
alp = set('abcdefghijklmnopqrstuvwxyz')
s = set(s)
if len(s & alp) == 26:
	ans = 'None'
else:
	lis = list(alp - s)
	lis.sort()
	ans = lis[0]
print(ans)


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


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


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


import re
s = str(input())
l = list('abcdefghijklmnopqrstuvwxyz')
for i in l:
	if not re.search(i, s):
		print(i)
		break
else:
	print('None')


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


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


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


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


S = input()
alpahbet = 'abcdefghijklmnopqrstuvwxyz'
string_list = []
ans = 'None'
for s in S:
	if s not in string_list:
		string_list.append(s)
for a in alpahbet:
	if a not in string_list:
		ans = a
		break
print(ans)


S = list(input())
alpha = 'abcdefghijklmnopqrstuvwxyz'
for a in alpha:
	if S.count(a) == 0:
		print(a)
		break
else:
	print('None')


s = input()
c = 'a'
while c <= 'z':
	if s.find(c) == -1:
		print(c)
		break
	if c == 'z':
		print('None')
	c = chr(ord(c) + 1)


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