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):
hoge=input()
flag=0
if(flag==0 and hoge.count("a")==0):
  print("a")
  flag=1

if(flag==0 and hoge.count("b")==0):
  print("b")
  flag=1

if(flag==0 and hoge.count("c")==0):
  print("c")
  flag=1

if(flag==0 and hoge.count("d")==0):
  print("d")
  flag=1

if(flag==0 and hoge.count("e")==0):
  print("e")
  flag=1

if(flag==0 and hoge.count("f")==0):
  print("f")
  flag=1

if(flag==0 and hoge.count("g")==0):
  print("g")
  flag=1

if(flag==0 and hoge.count("h")==0):
  print("h")
  flag=1

if(flag==0 and hoge.count("i")==0):
  print("i")
  flag=1

if(flag==0 and hoge.count("j")==0):
  print("j")
  flag=1

if(flag==0 and hoge.count("k")==0):
  print("k")
  flag=1

if(flag==0 and hoge.count("l")==0):
  print("l")
  flag=1

if(flag==0 and hoge.count("m")==0):
  print("m")
  flag=1

if(flag==0 and hoge.count("n")==0):
  print("n")
  flag=1

if(flag==0 and hoge.count("o")==0):
  print("o")
  flag=1

if(flag==0 and hoge.count("p")==0):
  print("p")
  flag=1

if(flag==0 and hoge.count("q")==0):
  print("q")
  flag=1

if(flag==0 and hoge.count("r")==0):
  print("r")
  flag=1

if(flag==0 and hoge.count("s")==0):
  print("s")
  flag=1

if(flag==0 and hoge.count("t")==0):
  print("t")
  flag=1

if(flag==0 and hoge.count("u")==0):
  print("u")
  flag=1

if(flag==0 and hoge.count("v")==0):
  print("v")
  flag=1

if(flag==0 and hoge.count("w")==0):
  print("w")
  flag=1

if(flag==0 and hoge.count("x")==0):
  print("x")
  flag=1

if(flag==0 and hoge.count("y")==0):
  print("y")
  flag=1

if(flag==0 and hoge.count("z")==0):
  print("z")
  flag=1
  
if(flag==0):
  print("None")

Solution (Python):
S = input()
Slist = []
for i in range(len(S)):
    Slist.append(ord(S[i]))
Slist = set(Slist)
i = 97
while i < 123:
    if i in Slist:
        i += 1
    else:
        print(chr(i))
        break
if i == 123:
    print('None')

Solution (Python):
S = input()

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

print(Ans)

Solution (Python):
s=input()
for i in range(97,123):
  if chr(i) in s:
    continue
  else:
    print(chr(i))
    break
else:
  print("None")

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

Solution (Python):
s = input()

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

Solution (Python):
S = list(input())
alphabet = list('abcdefghijklmnopqrstuvwxyz')
bucket = {}
for a in alphabet:
  bucket[a] = 0
for s in S:
  bucket[s] += 1
for a in alphabet:
  if bucket[a] == 0:
    print(a)
    break
else:
    print('None')

Solution (Python):
S = set(input())
result = list(set([chr(i) for i in range(97, 97+26)]) - S)
result.sort()
if result == []:
  print('None')
else:
  print(result[0])

Solution (Python):
s = str(input())
     
alphabet = "abcdefghijklmnopqrstuvwxyz"
     
ans ='None' 
for i in range(len(alphabet)):
        if not alphabet[i] in s:
                ans = alphabet[i]
                break
print(ans)

Solution (Python):
S = input()

an = ord("a")
cs = [False] * 26
for s in S:
    sn = ord(s)
    cs[sn-an] = True
    
for i, c in enumerate(cs):
    if not c:
        print(chr(i+an))
        break
else:
    print("None")

Solution (Python):



alp = [chr(i) for i in range(97,97+26)]
S = input()
l = list()
for i in S:
    l.append(i)
L =list(set(alp) - set(l))
if len(L)==0:
    print("None")
    exit()
L.sort()
print(L[0])

Solution (Python):
s=input()
alpha='abcdefghijklmnopqrstuvwxyz'
from collections import defaultdict
d=defaultdict(int)
for i in s:
    d[i]+=1
ans='None'
for a in alpha:
    if d[a]==0:
        ans=a
        break
print(ans)

Solution (Python):
abc_list = [chr(i) for i in range(97,97+26)]
input_list = list(input())
p_flag = 0
for i in abc_list:
    try:
        input_list.remove(i)
    except:
        print(i)
        p_flag = 1
        break
if(p_flag == 0):
    print("None")

Solution (Python):
a = 'abcdefghijklnmopqrstuvwxyz'

S = input()
for i in a:
    if i not in S:
        print(i)
        exit(0)
print('None')

Solution (Python):
s,d=input(),"abcdefghijklmnopqrstuvwxyz"
for i in range(26):
  if not d[i]in s:
    print(d[i])
    exit()
print("None")

Solution (Python):
def not_found(S: str)->str:
    for c in 'abcdefghijklmnopqrstuvwxyz':
        if c not in S:
            return c
    return 'None'


if __name__ == "__main__":
    S = input()
    ans = not_found(S)
    print(ans)

Solution (Python):
small=list('abcdefghijklmnopqrstuvwxyz')
s=list(input())
if len(set(s)&set(small))==26:
  print('None')
else:
  q=list(set(s)&set(small))
  for i in small:
    if i in q:
      continue
    else:
      print(i)
      exit()

Solution (Python):
s = input()
for i in "abcdefghijklmnopqrstuvwxyz":
    if s.find(i)==-1:
        print(i)
        exit()
else:
    print("None")

Solution (Python):
s=input()

L=list(s)

X=[0]*150
for i in L:
    X[ord(i)]+=1

ans='None'
for j in range(97,123):
    if X[j]==0:
        ans=chr(j)
        break

print(ans)

Solution (Python):
import string
S = sorted(list(set(input())))
for i, x in enumerate(string.ascii_lowercase):
    if len(S) <= i or S[i] != x:
        print(x)
        exit()
print("None")

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

Solution (Python):
def main():
    S = input()
    alph = [chr(i) for i in range(97, 97 + 26)]
    for _ in alph:
        if _ not in S:
            print(_)
            return
    print("None")

if __name__ == "__main__":
    main()

Solution (Python):
s = {c for c in input()}
for c in "abcdefghijklmnopqrstuvwxyz":
  if c not in s:
    print(c)
    break
else:
  print("None")

Solution (Python):
S = str(input())
M = "abcdefghijklmnopqrstuvwxyz"
result = M.strip(S)
char_len = len(result)
if char_len == 0:
        print("None")
else:
        print(result[0])

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

alpha = list('abcdefghijklmnopqrstuvwxyz')
c = 0

for i in range(len(alpha)):
	if alpha[i] not in s:
		print(alpha[i])
		c += 1              
		break
        
if c == 0:
	print('None')

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

Solution (Python):
s=input()
l=[]
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 x in alf:
    if str(x) in s:
        pass
    else:
        l.append(x)
if l==[]:
    print("None")
else:
    print(min(l))

Solution (Python):
S = input()
l = [chr(i) for i in range(ord('a'), ord('z')+1) if chr(i) not in S]

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

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

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

Solution (Python):
import math

def A():
    x, a, b = [int(x) for x in input().split()]
    if abs(x-a) < abs(x-b):
        print('A')
    else:
        print('B')
def B():
    S = input()
    alphabet = [False for x in range(26)]
    for char in S:
        i = ord(char) - ord('a')
        alphabet[i] = True
    printed = False
    for i in range(len(alphabet)):
        if alphabet[i] == False:
            print(chr(ord('a')+i))
            printed = True
            break
    if printed == False:
        print("None")
def C():
    n = int(input())
    three = 0

B()

Solution (Python):
import math

def A():
    x, a, b = [int(x) for x in input().split()]
    if abs(x-a) < abs(x-b):
        print('A')
    else:
        print('B')
def B():
    S = input()
    alphabet = [False for x in range(26)]
    for char in S:
        i = ord(char) - ord('a')
        alphabet[i] = True
    printed = False
    for i in range(len(alphabet)):
        if alphabet[i] == False:
            print(chr(ord('a')+i))
            printed = True
            break
    if printed == False:
        print("None")
B()

Solution (Python):

from collections import defaultdict
from collections import deque
from heapq import heappush, heappop
import sys
import math
import bisect
import random
import itertools
sys.setrecursionlimit(10**5)
stdin = sys.stdin
def LI(): return list(map(int, stdin.readline().split()))
def LF(): return list(map(float, stdin.readline().split()))
def LI_(): return list(map(lambda x: int(x)-1, stdin.readline().split()))
def II(): return int(stdin.readline())
def IF(): return float(stdin.readline())
def LS(): return list(map(list, stdin.readline().split()))
def S(): return list(stdin.readline().rstrip())
def IR(n): return [II() for _ in range(n)]
def LIR(n): return [LI() for _ in range(n)]
def FR(n): return [IF() for _ in range(n)]
def LFR(n): return [LI() for _ in range(n)]
def LIR_(n): return [LI_() for _ in range(n)]
def SR(n): return [S() for _ in range(n)]
def LSR(n): return [LS() for _ in range(n)]
mod = 1000000007
alp = [chr(ord('a') + i) for i in range(26)]

def A():
    x, a, b = LI()
    if abs(x - a) > abs(x - b):
        print("B")
    else:
        print("A")
    return


def B():
    s = S()
    s = set(s)
    for i in s:
        alp.remove(i)
    if len(alp) == 0:
        print("None")
    else:
        print(alp[0])
    return


def C():
    return


def D():
    return


def E():
    return


def F():
    return


def G():
    return


def H():
    return


if __name__ == '__main__':
    B()

Solution (Python):

from collections import defaultdict
from collections import deque
from heapq import heappush, heappop
import sys
import math
import bisect
import random
def LI(): return list(map(int, sys.stdin.readline().split()))
def I(): return int(sys.stdin.readline())
def LS():return list(map(list, sys.stdin.readline().split()))
def S(): return list(sys.stdin.readline())[:-1]
def IR(n):
    l = [None for i in range(n)]
    for i in range(n):l[i] = I()
    return l
def LIR(n):
    l = [None for i in range(n)]
    for i in range(n):l[i] = LI()
    return l
def SR(n):
    l = [None for i in range(n)]
    for i in range(n):l[i] = S()
    return l
def LSR(n):
    l = [None for i in range(n)]
    for i in range(n):l[i] = SR()
    return l
mod = 1000000007


"""
x,a,b = LI()
if abs(a-x) < abs(b-x):
    print("A")
else:
    print("B")
"""

s = S()
alp = list("qwertyuiopasdfghjklzxcvbnm")
alp.sort()
d = defaultdict(int)
for i in alp:
    d[i] += 1
for i in s:
    d[i] -= 1
for i in d.keys():
    if d[i] > 0:
        print(i)
        quit()
print("None")

"""
n = I()
d = defaultdict(int)
a = LI()
for i in a:
    d[i] += 1
d = list(d.items())
d.sort(key = lambda x:-x[0])
l = 0
k = 0
for i,j in d:
    if j >= 2:
        if l == 0:
            l = i
        else:
            k = i
        if j >= 4:
            k = i
    if k != 0:break
print(l*k)
"""


"""
n = I()
s1 = S()
s2 = S()
ans = 0
i = 0
key = 1
d = defaultdict(int)
while i < n:
    if s1[i] == s2[i]:
        if not d[s1[i]]:
            d[s1[i]] = 1
            if i == 0:
                ans = 3
            else:
                if key:
                    ans *= 2
        key = 1
    else:
        if not d[s1[i]] and not d[s2[i]]:
            d[s1[i]] = 1
            d[s2[i]] = 1
            if i == 0:
                ans = 6
            else:
                if key:
                    ans *= 2
                else:
                    ans *= 3
        if not d[s1[i]]:
            d[s1[i]] = 1
            if not key:
                ans *= 2
        if not d[s2[i]]:
            d[s2[i]] = 1
            if not key:
                ans *= 2
        key = 0
        i += 1
    i += 1
    ans %= mod
print(ans)
"""

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

Solution (Python):
S=str(input())
p=0
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 range(26):
    check=L[i] in S
    if check==True:
        p+=1
    else:
        print(L[i])
        break
if p==26:
    print('None')

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

Solution (Python):
S = list(set(input()))
alpha = "abcdefghijklnmopqrstuvwxyz"
S.sort
for i in range(len(alpha)):
    if alpha[i] not in S:
        print(alpha[i])
        exit()
print('None')

Solution (Python):
s = input()

for i in [chr(i) for i in range(97, 97+26)]:
    if s.count(i) == 0:
        print(i)
        exit()

print("None")

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

Solution (Python):
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"]
S = [i for i in input()]
for s in set(S):
    if s in alfa:
        alfa.remove(s)
if len(alfa)==0:
    print("None")
else:
    print(alfa[0])

Solution (Python):
S = input()

ans = None
for i in range(26):
    c = chr(i + ord('a'))
    if not c in S:
        ans = c
        break

print("None" if ans is None else ans)

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

Solution (Python):
S = set(input())
judge = True
albet=list("abcdefghijklmnopqrstuvwxyz")
for i in range(26):
    if not(albet[i] in S):
        judge = False
        break
        

if judge:
    print("None")
else:
    print(albet[i])

Solution (Python):
S = list(set(input()))
S.sort()
tmp = "abcdefghijklmnopqrstuvwxyz"
for i in range(0,len(tmp),1):
    if tmp[i] not in S:
        print(tmp[i])
        exit()
print("None")

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()
ans = "abcdefghijklmnopqrstuvwxyz"
for s in S:
  ans = ans.replace(s, "")
if(ans == ""):
  print("None")
else:
  print(ans[0])

Solution (Python):
def main():
    S = input()
    memo = [True] * 26
    a = ord('a')
    for c in S:
        memo[ord(c) - a] = False
    for i in range(26):
        if memo[i]:
            print(chr(i+a))
            return
    print("None")


if __name__ == "__main__":
    main()

Solution (Python):
a = set(list(input()))
ans = "None"
for i in range(ord("a"),ord("z")+1):
  if not chr(i) in a:
    ans = chr(i)
    break
print(ans)

Solution (Python):
s = input()

u = set(s)

if len(u) == 26:
	print("None")
	exit()
	
al = set("abcdefghijklmnopqrstuvwxyz")

print(sorted(al - u)[0])

Solution (Python):
tester = ["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()

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

print("None")

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

alph = [chr(i) for i in range(97, 97+26)]
l = list(set(alph) - set(s))
l.sort()
print(l[0]) if l != [] else print("None")

Solution (Python):
al = 'abcdefghijklmnopqrstuvwxyz'
s = input()
s = ''.join(sorted(list(set(s))))
for a in al:
  if a not in s:
    print(a)
    break
else:
  print('None')

Solution (Python):
al = 'abcdefghijklmnopqrstuvwxyz'
s = input()
s = ''.join(sorted(list(set(s))))
for a in al:
  if a not in s:
    print(a)
    break
else:
  print('None')

Solution (Python):
S = input()

isAppeared = [False]*(26)

for s in S:
    for i in range(97, 97+26):
        if s == chr( i ): isAppeared[i-97] = True

def printLetter( isAppeared ):
    for i in range(26):
        if isAppeared[i] == False:
            print( chr(i+97) )
            return
    print( 'None' )

printLetter( isAppeared )

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

Solution (Python):
s=input()
a="qwertyuioplkjhgfdsazxcvbnm"
kouho=[]
for i in a:
    if i not in s:kouho.append(i)
kouho=sorted(kouho)
print(kouho[0]) if len(kouho)!=0 else print("None")

Solution (Python):
"""AtCoder"""

S = input()

word = "abcdefghijklnmopqrstuvwxyz"

S = list(set(S))

S.sort()

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

print("None")

Solution (Python):

import sys
sys.setrecursionlimit(10**9)
input = sys.stdin.readline
s = input()
alpha = [chr(i) for i in range(97, 97+26)]
for c in alpha:
    if c in s:
        continue
    else:
        print(c)
        exit()
print("None")

Solution (Python):
S = list(input())
S = set(S)
S = list(S)
S.sort()
alphabet = list('abcdefghijklmnopqrstuvwxyz')

if(len(S) == 26):
  print('None')
else:
 for i in range(len(S)):
   alphabet.remove(S[i])
 print(alphabet[0])

Solution (Python):
import cmath
import numpy as np
import sys

INF=1e18
MOD=1e9+7

s=input()

alp = "abcdefghijklmnopqrstuvwxyz"

for c in alp:
    if c not in s:
        print(c)
        sys.exit()

print('None')

Solution (Python):
S = list(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']
for i in range(len(alpha)):
    if S.count(alpha[i])==0:
        print(alpha[i])
        exit()
print('None')

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

Solution (Python):
s=input()
abc="abcdefghijklmnopqrstuvwxyz"
for i in range(len(abc)):
    if (abc[i] in s)==False:
        print(abc[i])
        break
else:
    print("None")

Solution (Python):

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

Solution (Python):
s = input()
l = list('abcdefghijklmnopqrstuvwxyz')
for l_ in l:
    if l_ in s:
        continue
    else:
        print(l_)
        exit()
print('None')

Solution (Python):
s=list(input())
s=set(s)
s=list(s)
s.sort()
k=0
for i in range(26):
    if ord(s[i])-97!=i:
        if i==0:
            print("a")
            k=1
            break
        print(chr(ord(s[i-1])+1))
        break
    if i==25:
        print("None")
        k=1
        break
    if i==len(s)-1:
        print(chr(ord(s[-1])+1))
        break

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

for i in range(26):
    if check[i] not in s:
        print(check[i])
        break
    if i == 25:
        print('None')
        break

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

import string

s = set(sorted(list(s)))
t = set(list(string.ascii_lowercase))
remain = t - s
remain = list(remain)

if len(remain) > 0:
    print(min(remain))
else:
    print('None')

Solution (Python):
def notfound(n):
    bucket = {i:False for i in 'abcdefghijklmnopqrstuvwxyz'}
    for i in S:
        bucket[i]=True
    box = [k for k, v in bucket.items() if v == False]
    if box == []:
        return 'None'
    else:
        return sorted(box)[0]
S = input()
print(notfound(S))

Solution (Python):
dic = {}
for n in range(ord("a"), ord("z")+1):
    dic[chr(n)] = 0
s = input()
for l in s:
    dic[l] += 1

ans = "None"
for n in range(ord("a"), ord("z")+1):
    if dic[chr(n)] == 0:
        ans = chr(n)
        break
print(ans)

Solution (Python):
s = input()
xs = "abcdefghijklmnopqrstuvwxyz"
for a in s:
    xs = xs.replace(a, "", 1)
try:
    print(xs[0])
except:
    print("None")

Solution (Python):
S = set(input())
if len(S) == 26:
    print('None')
else:
    exclude = set('abcdefghijklmnopqrstuvwxyz')
    print(sorted(exclude - S)[0])

Solution (Python):
S = input()
S = list(S)
S = set(S)
S = list(S)
S.sort()
alpabet = 'abcdefghijklmnopqrstuvwxyz'
alphabet = list(alpabet)
count = 0
judge = 0

if(len(S) == 26):
  print('None')
else:
  for i in range(len(S)):
   count += 1
   if(S[i] != alphabet[i]):
      print(alphabet[i])
      judge = 1
      break
  if((count == len(S))and(judge==0)):
    print(alphabet[count])

Solution (Python):
S = raw_input()

for i in range(26):
  c = chr(ord('a')+i)
  if c not in S:
    print c
    break
else:
  print "None"

Solution (Python):
s = input()
baket = {}
answer = 'None'
for i in range(97, 97+26):
  baket[chr(i)] = 0
for j in s:
  baket[j] += 1
for i in range(97, 97+26):
  if baket[chr(i)] == 0:
    answer = chr(i)
    break
print(answer)

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

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

Solution (Python):
from statistics import mean, median,variance,stdev
import numpy as np
import sys
import math
import fractions
import itertools

def j(q):
    if q==1: print("Yes")
    else:print("No")
    exit(0)


def ct(x,y):
    if (x>y):print("")
    elif (x<y): print("")
    else: print("")

def ip():
    return int(input())

def swap(s,pos):
    s[pos]-=1
    s[pos+1]+=1
    print(s)
    if pos == n-2: return s
    if s[pos+2] == 0: swap(s,pos+1)
    return s

pl = 'abcdefghijklmnopqrstuvwxyz'






a = input()                                  






l = [pl[i] for i in range(26)]
for i in range(len(a)):
    if a[i] in l:
        p  = l.index(a[i])
        l.pop(p)
if len(l)==0:
    print("None")
else :
    print(l[0])

Solution (Python):

s=str(input())
s_l=set(s)
alphabet="abcdefghijklmnopqrstuvwxyz"
alpha_l=set(alphabet)
l=alpha_l-s_l
l=sorted(l)

if not l:
	print("None")
else:
	ans=l[0]
	print(l[0])

Solution (Python):
abc = set('abcdefghijklmnopqrstuvwxyz')
S = set(input())

print(sorted(abc - S)[0] if len(abc - S) != 0 else None)

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

Solution (Python):
s = input()
s = sorted(s)
a = "abcdefghijklmnopqrstuvwxyz"
for i in range(len(a)):
  if a[i] not in s:
    print(a[i])
    exit()
print("None")

Solution (Python):
import sys
s=input()
a='abcdefghijklmnopqrstuvwxyz'
for i in a:
    if s.find(i) == -1:
        print(i)
        sys.exit()
print('None')

Solution (Python):
s=list(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 x in a:
    if not x in s:
        print(x)
        exit()
print("None")

Solution (Python):
S = sorted(set(list(input())))
ABC = [chr(c) for c in range(ord('a'), ord('a') + 26)]
if S != ABC :
    for i in range(27) :
        try :
            if S[i] != ABC[i] :
                print(ABC[i])
                break
        except :
            print(ABC[i])
            break
else :
    print("None")

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

flag = False
for s in alp:
    if s not in S:
        print(s)
        flag = True
        break
if not flag:
    print('None')

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

for i in abc:
    if not s.count(i) == 1:
        ans = i
        break
else:
    ans = "None"

print(ans)

Solution (Python):
s = sorted(input())
alphabet = "abcdefghijklmnopqrstuvwxyz"
flag = 0
for t in alphabet:
  if s.count(t) == 0:
    print(t)
    break
  else:flag += 1

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

Solution (Python):

s = sorted(set(input()))
abc = 'abcdefghijklmnopqrstuvwxyz'
for a in abc:
    if s.count(a)==0:
        print(a)
        break
else:
    print('None')

Solution (Python):
S = input()
alp = 'abcdefghijklmnopqrstuvwxyz'
for s in S:
  if(s in alp):
    alp = alp.replace(s, '')
if(alp == ''):
  print('None')
else:
  print(alp[0])

Solution (Python):
s=[chr(i) for i in range(97, 97+26)]
a=sorted(list(input()))
for t in s:
  if t not in a:print(t);exit()
print("None")

Solution (Python):
s=input()

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

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

Solution (Python):
import string
S=input()
A=string.ascii_lowercase
B=list(set(A)-set(S))
B.sort()
print(B[0] if B else '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):
import sys
a=[0]*26
s=input()
for i in range(len(s)):
    a[ord(s[i])-97]=1
    
    if 0 not in a:
        print('None')
        sys.exit(0)
for i in range(26):
    
    if a[i]==0:
        print(chr(i+97))
        break

Solution (Python):
import string
S = list(input().rstrip())
S.sort()
t = set(S)
if len(t) == 26: print('None')
else:
    for u in string.ascii_lowercase:
        if not u in t:
            print(u)
            break

Solution (Python):
s = set(input())
on = set("abcdefghijklmnopqrstuvwxyz")
try:
  print(min(on-s))
except ValueError:
  print("None")

Solution (Python):
s = sorted(set(list(str(input()))))
t = "abcdefghijklmnopqrstuvwxyz"
l = []
if len(s) == 26:
    print("None")
for i in range(26):
    if t[i] not in s:
        print(t[i])
        break

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

Solution (Python):
s = set(input())
t = set('abcdefghijklmnopqrstuvwxyz')
ans = sorted(list(s ^ t))
print(ans[0] if len(ans) != 0 else 'None')

Solution (Python):
alphabets = [i for i in range(ord('a'), ord('z') + 1)]
lists = [ord(i) for i in input()]
lists=list(set(lists))
lists.sort()
for i in range(len(alphabets)):
    if i+1>=len(lists) and i!=25:
        if alphabets[i] == lists[i]:
            print(chr(alphabets[i+1]))
            exit(0)
    if alphabets[i]!=lists[i]:
        print(chr(alphabets[i]))
        exit(0)


print('None')

Solution (Python):
s=input()
for a in [chr(i) for i in range(97, 97+26)]:
  if a not in s:
    print(a)
    exit()
print('None')

Solution (Python):
s = input()

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

Solution (Python):
A=input()
chars = list(A)

i=97
while chars.count(chr(i))>0 and i<(97+26):
    i=i+1
if i<(97+26):
    print(chr(i))
else:
    print("None")

Solution (Python):
S = input()

unko =["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 unko:
    if S.count(i) == 0:
        print(i)
        exit()

print("None")

Solution (Python):
A=input()
B=list("abcdefghijklmnopqrstuvwxyz")
for i in range(26):
  if not B[i]  in A:
    print(B[i])
    exit()
print("None")

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

Solution (Python):
str_s = input()

all = set([chr(i) for i in range(97, 97+26)])
inputs = set(list(str_s))

diff = all.difference(inputs)

if len(diff) > 0:
    print(sorted(list(diff))[0])
else:
    print("None")

Solution (Python):
import string
x = input()
def func(x):
    diff = set(string.ascii_lowercase)- set(x)
  
    if diff:
        print(sorted(diff)[0])
    else:
        print('None')
        
func(x)

Solution (Python):
s=input()

al="abcdefghijklmnopqrstuvwxyz"
ishoge = False

for a in al:
    isExist = False
    for char in s:
        if a==char:
            isExist = True
            break
            
    if not isExist:
        print(a)
        ishoge = True
        break
        
if not ishoge:
    print("None")

Solution (Python):
S = input()
s = set()
table = "abcdefghijklmnopqrstuvwxyz"
for x in S:
  s.add(x)
v = 0
for x in table:
  if x not in s:
    print(x)
    v = 1
    break
  else:
    pass
if v == 0:
  print("None")

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

for a in 'abcdefghijklmnopqrstuvwxyz':
    if a not in s:
        print(a)
        break
else:
    print('None')

Solution (Python):
*s, = map(ord, list(input()))
s = set(s)

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

Solution (Python):
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"]
s = str(input())
for i in range(len(s)):
    if s[i] in alphabet:
        m = alphabet.index(s[i])
        del alphabet[m]
    if alphabet == []:
        print("None")
        exit()
print(alphabet[0])

Solution (Python):
import string


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


if __name__ == '__main__':
    main()

Solution (Python):
import string
n = input()
s = set(string.ascii_lowercase) - set(n)

if len(s) == 0:
    print('None')
else:
    print(min(s))

Solution (Python):
s = list(input())
s_ = list('abcdefghijklmnopqrstuvwxyz')

s = list(set(s))
s.sort()

for i in range(len(s)):
    if s[i] != s_[i]:
        print(s_[i])
        exit()
if len(s) < 26: print(s_[len(s)])
else: print('None')

Solution (Python):
s = set(input())
d = set('abcdefghijklmnopqrstuvwxyz')

if d - s == set():
    print('None')
else:
    print(sorted(d - s)[0])

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

if len(s) == 26:
        print('None')

else:
    for i in s:
        d.remove(i)
    print(d[0])

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

Solution (Python):
alpha = [chr(ord('a') + i) for i in range(26)]
n = list(input())
is_not_exist = True
for i in alpha:
    if i not in n:
        print(i)
        is_not_exist = False
        break
if is_not_exist:
    print(None)

Solution (Python):
import sys





S = str(input())
ll = list("abcdefghijklmnopqrstuvwxyz")
for al in ll:
    if not (al in S):
        print(al)
        sys.exit()
print("None")

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

alphabet_set = set(alphabet)
nokori = alphabet_set - s_set
nokori_sort = sorted(list(nokori))
if len(nokori_sort) == 0:
	print("None")
	exit()
print(nokori_sort[0])

Solution (Python):
s=input()
s=set(s)
t=[ord(i) for i in s]
t.sort()

flag=0

for i in range(97,122+1):
  if t.count(i)==0 and flag==0:
    ans=i
    flag=1

print(chr(ans) if flag==1 else 'None')