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

Solution (Python):






import sys
input = sys.stdin.readline

import math

import bisect
from collections import defaultdict
from collections import deque
from functools import lru_cache






MOD = 10**9+7
INF = float('inf')






def inputI(): return int(input().strip())
def inputS(): return input().strip()
def inputIL(): return list(map(int,input().split()))
def inputSL(): return list(map(str,input().split()))
def inputILs(n): return list(int(input()) for _ in range(n))
def inputSLs(n): return list(input().strip() for _ in range(n))
def inputILL(n): return [list(map(int, input().split())) for _ in range(n)]
def inputSLL(n): return [list(map(str, input().split())) for _ in range(n)]


def Yes(): print("Yes"); return
def No(): print("No"); return


def inv(n): return pow(n, MOD-2, MOD)


kaijo_memo = []
def kaijo(n):
  if(len(kaijo_memo) > n):
    return kaijo_memo[n]
  if(len(kaijo_memo) == 0):
    kaijo_memo.append(1)
  while(len(kaijo_memo) <= n):
    kaijo_memo.append(kaijo_memo[-1] * len(kaijo_memo) % MOD)
  return kaijo_memo[n]

gyaku_kaijo_memo = []
def gyaku_kaijo(n):
  if(len(gyaku_kaijo_memo) > n):
    return gyaku_kaijo_memo[n]
  if(len(gyaku_kaijo_memo) == 0):
    gyaku_kaijo_memo.append(1)
  while(len(gyaku_kaijo_memo) <= n):
    gyaku_kaijo_memo.append(gyaku_kaijo_memo[-1] * pow(len(gyaku_kaijo_memo),MOD-2,MOD) % MOD)
  return gyaku_kaijo_memo[n]

def nCr(n,r):
  if(n == r):
    return 1
  if(n < r or r < 0):
    return 0
  ret = 1
  ret = ret * kaijo(n) % MOD
  ret = ret * gyaku_kaijo(r) % MOD
  ret = ret * gyaku_kaijo(n-r) % MOD
  return ret


def factorization(n):
  arr = []
  temp = n
  for i in range(2, int(-(-n**0.5//1))+1):
    if temp%i==0:
      cnt=0
      while temp%i==0: 
        cnt+=1 
        temp //= i
      arr.append([i, cnt])
  if temp!=1:
    arr.append([temp, 1])
  if arr==[]:
    arr.append([n, 1])
  return arr


def make_divisors(n):
  divisors = []
  for i in range(1, int(n**0.5)+1):
    if n % i == 0:
      divisors.append(i)
      if i != n // i: 
        divisors.append(n//i)
  return divisors


def lcm(a, b):
    return a * b // gcd (a, b)


def count_bit(n):
  count = 0
  while n:
    n &= n -1
    count += 1
  return count


def Base_10_to_n(X, n):
  if X//n:
    return Base_10_to_n(X//n, n)+[X%n]
  return [X%n]

def Base_n_to_10(X, n):
  return sum(int(str(X)[-i])*n**i for i in range(len(str(X))))


def int_log(n, a):
  count = 0
  while n>=a:
    n //= a
    count += 1
  return count




s = inputS()

alpha = "abcdefghijklmnopqrstuvwxyz"

for l in alpha:
  if l not in s:
    print(l)
    exit()
print("None")

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

Solution (Python):
s=input()
alp="abcdefghijklmnopqrstuvwxyz"
c=0
for i in alp:
    if i not in s:
        print(i)
        c+=1
        break
if c==0:
    print("None")

Solution (Python):
from string import ascii_lowercase

S = set(input())

ans = list(set(ascii_lowercase) - S)
ans.sort()
if len(ans):
    print(ans[0])
else:
    print("None")

Solution (Python):
S = input()
s = ord("a")

for i in range(26):
  if chr(s) in S:
    s=s+1
    if i==25:
      print("None")
    continue
  else:
    print(chr(s))
    break

Solution (Python):
s=input()
ok=True
for i in range(97,97+26):
    if chr(i) not in s:
        ok=False
        print(chr(i))
        break
if ok:
    print("None")

Solution (Python):
s = input()

alphabet = 'abcdefghijklmnopqrstuvwxyz'

ans = set(s) ^ set(alphabet)
print(sorted(list(ans))[0]) if len(ans) != 0 else print('None')

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

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

Solution (Python):
S = input()

al = [chr(i) for i in range(ord('a'), ord('z')+1)]

for j in range(len(S)):
    if S[j] in al:
        al.remove(S[j])
if al:
    print(al[0])
else:
    print('None')

Solution (Python):
S = list(input())
S = sorted(list(set(S)))
i = 97
for s in S:
    if ord(s) != i:
        print(chr(i))
        exit()
    i += 1
if len(S) < 26:
    print(chr(i))
else:
    print('None')

Solution (Python):
S = str(input())
alp = "abcdefghijklmnopqrstuvwxyz"
st = 'None'
for i in range(26):
  if alp[i] not in S:
    st = alp[i]
    break
print(st)

Solution (Python):
s = input()
a = 'abcdefghijklmnopqrstuvwxyz'
cnt = []
for i in range(26):
  if a[i] in s:
    continue
  else:
    cnt.append(a[i])
if cnt == []:
    print('None')
else:
    print(cnt[0])

Solution (Python):
s = list(input())
s = list(set(s))
for i in range(ord("a"),ord("z")+1):
    if s.count(chr(i)) != 1:
        print(chr(i))
        exit()
print("None")

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

ans = "None"
for j in [chr(i) for i in range(97,97+26)]:
        if j not in S:
            ans = j
            break

print(ans)

Solution (Python):
S = set(list(input()))
L = "abcdefghijklmnopqrstuvwxyz"

ans = "None"
if len(S) != 26:
    for j in L:
        if j not in S:
            ans = j
            break

print(ans)

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

for t in alphabet:
    if t not in s:
        print(t)
        break
    if t == "z":
        print("None")

Solution (Python):
S = list(input())
S = set(S)
ans = "None"
for i in range(26):
  if not(chr(97+i) in S):
    ans = chr(97+i)
    break
print(ans)

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):
s = input()
for x in [chr(i) for i in range(97, 97+26)]:
    if x not in s:
        print (x)
        exit ()
print ("None")

Solution (Python):
S = input()
alphabet_list = [chr(i) for i in range(97, 97 + 26)]
flag = 0
for moji in alphabet_list:
    if moji not in S:
        print(moji)
        flag = 1
        break
if not flag:
    print("None")

Solution (Python):
S = input()
S = sorted(set(S))
A = "abcdefghijklmnopqrstuvwxyz"
for i in range(len(A)):
  if len(A) == len(S):
    print("None")
    break
  if A[i] != S[i]:
    print(A[i])
    break
  if i == len(S) - 1:
    print(A[i + 1])
    break

Solution (Python):
import sys
S = list(set(list(sys.stdin.readline().strip())))

for s in range(ord('a'), ord('z') + 1):
    if chr(s) not in S:
        print(chr(s))
        exit()
print("None")

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

a = list(set(s)^set(al))

if len(a) == 0:
    print('None')
else:
    a.sort()
    print(a[0])

Solution (Python):
from sys import exit

ii = lambda : int(input())
mi = lambda : map(int,input().split())
li = lambda : list(map(int,input().split()))

s = input()

sa = sorted(s)
al="abcdefghijklmnopqrstuvwxyz"
cnt=0

for i in range(len(s)):
    if sa[i] == al[cnt]:
        cnt+=1
    if cnt == 26:
        print("None")
        exit()

print(al[cnt])

Solution (Python):
S=input()
L=list(set(S))
l=list('abcdefghijklmnopqrstuvwxyz')
for x in L:
  l.remove(x)
  if len(l)==0:
    print('None')
    exit()
else:
  print(l[0])

Solution (Python):
s = list(input())
t = ['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(t)):
    if s.count(t[i]) == 0:
        print(t[i])
        exit()
print('None')

Solution (Python):
S = list(input())
alphabet = []

for i in range(26):
    alphabet.append(chr(i+97))

x = set(S)

for i in alphabet:
    if i not in x:
        print(i)
        exit()

print('None')

Solution (Python):
n=input()

l=[chr(ord('a') + i) for i in range(26)]
for k in l:
    if k not in n:
      print(k)
      exit()
print("None")

Solution (Python):
import sys
import math
import bisect

def solve(s):
    for i in range(26):
        c = chr(i + ord('a'))
        if c not in s:
            return c
    return None

def main():
    ans = solve(input())
    if ans:
        print(ans)
    else:
        print('None')

if __name__ == "__main__":
    main()

Solution (Python):
S = list(input())
s = sorted(list(set(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']

if s == l:
    ans = 'None'
else:
    for i in range(26):
        if (len(s) - 1) < i:
            ans = l[i]
            break
        elif s[i] != l[i]:
            ans = l[i]
            break

print(ans)

Solution (Python):
s=str(input())
s=list(s)
s=set(s)
s=list(s)
s.sort()
a="abcdefghijklmnopqrstuvwxyz"
a=list(a)
i=0
count=0
while i<len(s):
  if s[i]!=a[i]:
    print(a[i])
    break
  i=i+1
  count=count+1
else:
  if count==26:
    print("None")
  else:
    print(a[i])

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

    bucket = list('' for _ in range(26))

    for char in s:
        bucket[ord(char) - 97] = char
    
    try:
        print(chr(bucket.index('') + 97))
    except:
        print('None')

if __name__ == '__main__':
    main()

Solution (Python):

S = sorted(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 A:
    if i not in S:
        print(i)
        exit()
else:
    print('None')

Solution (Python):
n=input()

la=[chr(ord('a') + i) for i in range(26)]
for i in la:
    if i not in n:
      print(i)
      exit()
print("None")

Solution (Python):
import sys
S = input()
T = []
B = []
A = 'abcdefghijklmnopqrstuvwxyz'
for i in range(len(S)):
   T.append(S[i])
for i in range(len(A)):
   B.append(A[i])
for i in B:
    if not i in T:
        print(i)
        sys.exit()
print('None')

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"]

s=input()
l=len(s)

for i in range(l):
    if s[i] in alp:
        alp.remove(s[i])

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

Solution (Python):
S = input()
alp = []
for i in range(97,123):
    alp.append(chr(i))
for s in S:
    if s in alp:
        alp.remove(s)
if len(alp) == 0:
    print('None')
else:
    print(alp[0])

Solution (Python):
from collections import Counter

s = input()

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

counter = Counter(list(s))

for a in alphabets:
    if counter[a] == 0:
        print(a)
        exit()

print("None")

Solution (Python):
import math
def py():
    print("Yes")
def pn():
    print("No")
def iin():
    x = int(input())
    return x

neko = 'None'

s = input()
alfa = ['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 x in alfa:
    if not(x in s):
        neko = x
        break
print(neko)

Solution (Python):
S = list(input())
All = list("abcdefghijklmnopqrstuvwxyz")
res = list(set(S) ^ set(All))
if len(res) > 0:
    print(res[0])
else:
    print("None")

Solution (Python):
S = input().strip()

alpha = [chr(i) for i in range(ord("a"), ord("z")+1)]
d = {}
for i in alpha:
    d[i] = 0

for i in S:
    d[i] += 1

is_none = True
for i in alpha:
    if d[i] == 0:
        is_none = False
        print(i)
        break

if is_none:
    print("None")

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

li = list({chr(ord('a') + i) for i in range(26)} - S)
li = sorted(li)

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

Solution (Python):
s = input()

n = len(s)
l = list()

for a in range(0, n):
    l.append(s[a])

l.sort()
l = list(set(l))
n = len(l)

if n == 26:
    print('None')
else:
    for a in range(0, n):
        l[a] = int(ord(l[a]))
    for a in range(97, 123):
        if a not in l:
            print(chr(a))
            break

Solution (Python):
import sys

S = input()
alphabet = list(map(chr, range(97, 97+26)))

for char in alphabet:
    if not char in S:
        print(char)
        sys.exit()

print('None')

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"]

s=input()
l=len(s)

for i in range(l):
    if s[i] in alp:
        alp.remove(s[i])

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

Solution (Python):
import sys
import string

input = sys.stdin.readline

def main():
    ans = 'None'
    S = input()
    s = string.ascii_lowercase
    for i in s:
        if i not in S:
            ans = i
            break
    print(ans)

if __name__ == '__main__':
    main()

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

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

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

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

for l in lists:
    if S.find(l) == -1:
        print(l)
        exit()
print('None')

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

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

print("None")

Solution (Python):
s = input()

t = "abcdefghijklmnopqrstuvwxyz"
s = sorted(set(s))
for tt in t:
    if tt not in s:
        print(tt)
        exit()
print('None')

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

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

Solution (Python):
S = input()

for i in range(ord("a"), ord("z") + 1):
  s = chr(i)
  if S.count(s) == 0:
    print(s)
    exit()
print("None")

Solution (Python):
s = input()
a = [True for _ in range(26)]
for si in s:
  a[ord(si)-97] = False
for i in range(26):
  if a[i]:
    print(chr(i+97))
    exit()
print('None')

Solution (Python):
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"]
A=list(input())
A=set(A)
for i in range(26):
    if l[i] in A:
        pass
    else:
        print(l[i])
        exit()
print("None")

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

Solution (Python):


import sys
input= lambda: sys.stdin.readline().rstrip()
sys.setrecursionlimit(10**9)
def pin(type=int):return map(type,input().split())
def tupin(t=int):return tuple(pin(t))
def lispin(t=int):return list(pin(t))


def resolve():
    S=input()
    a=ord("a")

    for i in range(26):
        temp=a+i
        for s in S:
            if s==chr(temp):break
        else:print(chr(temp));return
    print("None")
                

            
        

resolve()

Solution (Python):
def resolve():
    import string
    s = str(input())
    alpha = set(string.ascii_lowercase)
    for a in s:
        alpha.difference_update(a)

    try:
        print(sorted(alpha)[0])
    except IndexError:
        print('None')


resolve()

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


if __name__ == "__main__":
    main()

Solution (Python):
S1 = set(list('abcdefghijklmnopqrstuvwxyz'))
S2 = set(list(input()))
S3 = S1-S2
if len(S3) == 0:
    print('None')
else:
    print(sorted(list(S3))[0])

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

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

chr = 'abcdefghijklmnopqrstuvwxyz'

res =""

for i in range(len(chr)):
    
    if chr[i] not in S:
        res = chr[i]
        break
    
if res =="":
    print("None")
else:
    print(res)

Solution (Python):
S=input()
alf=['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(S)):
  if S[i] in alf:
    alf.remove(str(S[i]))
  
if len(alf)>0:
  print(alf[0])
  exit()
print('None')

Solution (Python):
import string

S = input()
l = []
char = string.ascii_lowercase

for c in char:
    if not c in S:
        l.append(c)

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

Solution (Python):
s = input()
a = 'abcdefghijklmnopqrstuvwxyz'
hoge_list = list(a)
s_list = list(s)
c = 0
s_s_list = set(s_list)
for hoge in hoge_list:
  if not hoge in s_s_list:
    c += 1
    print(hoge)
    break
if c == 0:
  print('None')

Solution (Python):
import sys
from string import ascii_lowercase as AL

input = sys.stdin.readline


def main():
    S = input().rstrip()

    counter = {}
    for s in AL:
        counter[s] = 0

    for s in S:
        counter[s] += 1

    ans = ""
    for s in AL:
        if counter[s] == 0:
            ans = s
            break
    if not ans:
        ans = "None"

    print(ans)


if __name__ == "__main__":
    main()

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

alpha = [False] * 26
for i in range(len(s)):
    alpha[ord(s[i]) - 97] = True

for i in range(len(alpha)):
    if not alpha[i]:
        print(chr(97 + i))
        break
    if i == 25:
        print("None")

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

flg = True

for i in range(ord('a'), ord('z') + 1):
    if S.count(chr(i)) == 0:
        flg = False
        ans = chr(i)
        break

print("None" if flg else ans)

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

for char in alphabets:
  if char not in s:
    print(char)
    break
else:
  print('None')

Solution (Python):

import sys
import math
from bisect import bisect_left, bisect_right, insort_left, insort_right
from collections import defaultdict, deque, Counter
from copy import deepcopy
from fractions import gcd
from functools import lru_cache, reduce
from heapq import heappop, heappush
from itertools import accumulate, groupby, product, permutations, combinations, combinations_with_replacement
from math import ceil, floor, factorial, log, sqrt, sin, cos
from operator import itemgetter
from string import ascii_lowercase, ascii_uppercase, digits
sys.setrecursionlimit(10**7)
rs = lambda: sys.stdin.readline().rstrip()
ri = lambda: int(rs())
rf = lambda: float(rs())
rs_ = lambda: [_ for _ in rs().split()]
ri_ = lambda: [int(_) for _ in rs().split()]
rf_ = lambda: [float(_) for _ in rs().split()]
INF = float('inf')
MOD = 10 ** 9 + 7
PI = math.pi

S = list(rs())
for i in ascii_lowercase:
    if not i in S:
        print(i)
        exit()
print('None')

Solution (Python):
a = [chr(ord("a") + i) for i in range(26)]
s = sorted(set(input()))

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

print("None")

Solution (Python):
import sys
S = sorted(set(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']

for i in range(len(alp)):
  if (i>len(S)-1):
    print(alp[i])
    sys.exit()
  elif S[i] != alp[i]:
    print(alp[i])
    sys.exit()

print('None')

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

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

print("None")

Solution (Python):
s = set(list("abcdefghijklmnopqrstuvwxyz"))-set(list(input()))
if len(s)==0:
  print("None")
else:
  print(min(s))

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

if "".join(s) == a:
    print("None")
    exit()
else:
    for i in s:
        a = a.replace(i, "")

print(a[0])

Solution (Python):
a=[chr(ord('a') + i) for i in range(26)]
s=input()
ans=[]
for k in a:
    if k not in s:
        ans.append(k)
if not ans:
    print("None")
else:
    print(min(ans))

Solution (Python):
S = input()

s = set([c for c in S])

x = 'abcdefghijklmnopqrstuvwxyz'

for a in s:
    x = x.replace(a, '')
if len(x) == 0:
    print('None')
else:
    print(x[0])

Solution (Python):
s=input()
a="abcdefghijklmnopqrstuvwxyz"
for i in range(26):
  if a[i] in s:
    continue
  else:
    print(a[i])
    exit()
print("None")

Solution (Python):
S = sorted(list(set(list(input()))))
alpha = [int(i) for i in range(0,27)]
if len(S) == 26:
    print("None")
else:
    for i in range(0, 27):
        if chr(alpha[i]+97) not in S:
            print(chr(alpha[i]+97))
            break

Solution (Python):
import sys
import heapq, math
from itertools import zip_longest, permutations, combinations, combinations_with_replacement
from itertools import accumulate, dropwhile, takewhile, groupby
from functools import lru_cache
from copy import deepcopy

S = input()
use = [False] * 26

for s in S:
    use[ord(s) - 97] = True

def ans():
    for i in range(26):
        if not use[i]:
            return chr(i + 97)

    return "None"

print(ans())

Solution (Python):
S = input()
S = set(S)
lst = [chr(i) for i in range(ord('a'), ord('z') + 1)]
for i in lst:
    if i not in S:
        print(i)
        exit(0)
print('None')

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

Solution (Python):
s = input()

if len(set(s)) == 26:
  print("None")
else:
  for c in "abcdefghijklmnopqrstuvwxyz":
    if not c in set(s):
      break
  
  print(c)

Solution (Python):
S=input()

ans='None'
i=97
while i<=122:
    if chr(i)not in S:
        ans=chr(i)
        break
    i+=1

print(ans)

Solution (Python):
s = sorted(set(input()))
for i in range(97, 123):
    for j in range(len(s)):
        if s[j] == chr(i):
            break
    else:
        print(chr(i))
        break
else:
    print('None')

Solution (Python):
s = input().strip()

d = {
    '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,
}

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

for c in sorted(d.keys()):
    if d[c] == 0:
        print(c)
        exit(0)

print('None')

Solution (Python):
import string

stg = input()
res = "None"
lets = {stg[0]}
if len(stg)>1:
  for i in range(1,len(stg)):
    lets.add(stg[i])
alpha = list(string.ascii_lowercase)
for i in alpha:
  if i not in lets:
    res = i
    break
print(res)

Solution (Python):
s = list(input())
for c in range(26):
    X = chr(ord('a') + c)
    if X not in s:
        print(X)
        exit()

print('None')

Solution (Python):
S=set(input())
arr=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'])
result=arr-S
if len(result)==0:
  print('None')
else:
  print(sorted(result)[0])

Solution (Python):
from sys import stdin


if __name__ == "__main__":
    _in = [_.rstrip() for _ in stdin.readlines()]
    S =     _in[0]   
    
    s_arr = [s for s in S]
    s_arr = list(set(s_arr))
    if len(s_arr) == 26:
        s = 'None'
    else:
        s_arr.sort()
        for i in range(26):
            if len(s_arr) <= i:
                s = chr(i+97)
                break
            elif s_arr[i] != chr(i+97):
                s = chr(i+97)
                break
    
    print(s)

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

Solution (Python):
import string

def resolve():
    
    s = input()
    
    
    
    
    
    
    
    

    alpha = string.ascii_lowercase
    A = [alpha[i] for i in range(len(alpha))]

    for x in A:
        if x not in s:
            print(x)
            break
        if x == "z":
            print("None")

resolve()

Solution (Python):
import sys
input = sys.stdin.readline
sys.setrecursionlimit(10**7)
from collections import Counter, deque
from collections import defaultdict
from itertools import combinations, permutations, accumulate, groupby, product
from bisect import bisect_left,bisect_right
from heapq import heapify, heappop, heappush
from math import floor, ceil,sqrt
from operator import itemgetter
def I(): return int(input())
def MI(): return map(int, input().split())
def LI(): return list(map(int, input().split()))
def LI2(): return [int(input()) for i in range(n)]
def MXI(): return [[LI()]for i in range(n)]
def printns(x): print('\n'.join(x))
def printni(x): print('\n'.join(list(map(str,x))))
inf = 10**17
mod = 10**9 + 7


alphasm=[chr(i) for i in range(97, 97+26)]
s=input().rstrip()
s=list(s)
u=sorted(list(set(s)))
ans=0
for al in alphasm:
    if not (al in u):
        ans=al
        break
print(ans if ans!=0 else "None")

Solution (Python):
S = list(input())
A = [chr(ord('a') + i) for i in range(26)]
cnt = 0
for i in A:
  if i in S:
    continue
  else:
    print(i)
    cnt = 1
    break
if cnt == 0:
  print('None')

Solution (Python):
s = input()
g1 = set("abcdefghijklmnopqrstuvwxyz")
g2 = set(s)
a = g1 -g2
if a == set():
    print('None')
    

else:
    print(sorted(list(a))[0])

Solution (Python):
def main():
    S = input()
    st = list(map(lambda x: chr(97+x), range(26)))
    for ss in S:
        if ss in st:
            st.remove(ss)

    print(st[0] if len(st) > 0 else 'None')


if __name__ == "__main__":
    main()

Solution (Python):
a=[0 for i in range(26)]
s=input()
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))
        break
else:
    print(None)

Solution (Python):
from string import ascii_lowercase as abc
s = input()
if set(abc) == set(s):
    print('None')
else:
    print(sorted(list(set(abc)-set(s)))[0])

Solution (Python):
def resolve():
	s = input()
	t = [True]*26
	for i in s:
		t[ord(i) - 97] = False
	for i in range(26):
		if t[i]:
			print(chr(i+97))
			return
	print('None')
resolve()

Solution (Python):
S = input()
S = sorted(set(S))
S.append("0")
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 True:
    if S[i] != B[i]:
        print(B[i])
        break
    i += 1
    if i == 26:
        print("None"  )
        break

Solution (Python):
S = input()
List = []
for i in range(len(S)):
  tmp = S[i]
  List.append(tmp)
List = list(set(List))
num2alpha = lambda c: chr(c+96)
for k in range(1,27):
  if(num2alpha(k) in List):
    pass
  else:
    print(num2alpha(k))
    exit()
print("None")

Solution (Python):
import string

s = input()

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


for x in lall:
    if x not in s:
        print(x)
        exit()
print("None")

Solution (Python):
S=input()
alphabet=[chr(i) for i in range(97, 97+26)]
ans=[]
for a in alphabet:
  if a not in S:
    ans.append(a)
if not ans:
  print('None')
else:
  print(ans[0])

Solution (Python):
s=sorted(set(input()))
for i in range(len(s)):
    s[i]=ord(s[i])
s.append("*")
x=0
a=97
while s[x]==a:
    x+=1
    a+=1
print(chr(a) if a<123 else None)

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

Solution (Python):
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"]
moji = str(input())
ans = "z"
cnt = 0
for i in range(26):
  if lst[i] not in moji and lst[i] <= ans:
    ans = lst[i]
    cnt += 1
if cnt > 0:
  print(ans)
elif cnt==0:
  print("None")

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

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

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

for i in range(ord('a'),ord('z')+1):
  if not chr(i) in sset:
    print(chr(i))
    break
else:
  print("None")

Solution (Python):
N=list(input())
a=[chr(i) for i in range(97, 97+26)]
d={a[i]:0 for i in range(len(a))}

for x in N:
    d[x]+=1
i=0
while i<=len(a)-1:
    if d[a[i]]==0:
        print(a[i])
        break
    i+=1
else:
    print('None')

Solution (Python):
import sys
def input(): return sys.stdin.readline().strip()

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

Solution (Python):
s =list(input())
syurui = []
for i in s:
    if i not in syurui:
        syurui.append(i)
ans = sorted(syurui)
if len(ans)==26:
    print('None')
else:
    num = [97+i for i in range(26)]
    for i in num:
        if chr(i) not in ans:
            answer = i
            break
    print(chr(i))

Solution (Python):
s=set(list(input()))
if len(s)==26:print("None")
else:
    import string
    f="abcdefghijklmnopqrstuvwxyz"
    for i in list(f):
        if i not in s:print(i);exit()

Solution (Python):
s = input()
flag = [True]*26
for t in s:
    flag[ord(t)-ord('a')] = False
for i in range(26):
    if flag[i]:
        print(chr(i+ord('a')))
        break
else:
    print('None')

Solution (Python):
import sys
def input(): return sys.stdin.readline().strip()

def resolve():
    s=input()
    l='abcdefghijklmnopqrstuvwxyz'
    ans='None'
    for i in l:
        if i not in s:
            ans=i
            break
    print(ans)
resolve()

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

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

Solution (Python):
S = input()

import sys
for i in range(26):
    ans = chr(ord('a') + i)
    if not(ans in S):
        print(ans)
        sys.exit()
ans = "None"
print(ans)

Solution (Python):
import string

S = list(map(str, input()))
s = sorted(set(S))
count = 0

alphabet = string.ascii_lowercase

if len(s) == len(alphabet):
    print("None")

else:
    for i in range(len(s)):
        if s[i] != alphabet[i]:
            print(alphabet[i])
            break
        else:
            count += 1

if count == len(s):
    print(alphabet[len(s)])

Solution (Python):
S=input()
count={'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}
for i in range(len(S)):
    count[S[i]]+=1

ans=[]
for key, value in count.items():
    if value==0:
        ans.append(key)

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