Buggy python submission (verdict: Runtime Error):
S = list(set(input()))
S.sort()
flag = False
tmp = "abcdefghijklmnopqrstuvwxyz"
for i in range(0,len(tmp),1):
    if str(S[i]) != str(tmp[i]):
        print(tmp[i])
        break
if flag != False:
    print("None")


Fixed submission:
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")

Buggy python submission (verdict: Runtime Error):
S = list(input())
li = list(set(S))
li.sort()
ascii_lowercase = list("abcdefghijklmnopqrstuvwxyz")
for i in range(0,len(S),1):
  if li[i] != ascii_lowercase[i]:
    print(ascii_lowercase[i])
    break
  elif li == ascii_lowercase:
    print("None")
    break

Fixed submission:
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")

Buggy python submission (verdict: Runtime Error):
S = input()
li = [i for i in S]
li_2 = list(set(li))
li_2.sort()
ascii_lowercase = "abcdefghijklmnopqrstuvwxyz"
ascii_lowercase_list = list(ascii_lowercase)
for k in range(0,len(S),1):
  if li_2[k] != ascii_lowercase_list[k]:
    print(ascii_lowercase_list[k])
    break
  elif li_2 == ascii_lowercase_list:
    print("None")
    break

Fixed submission:
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")

Buggy python submission (verdict: Runtime Error):
S = str(input())
li = [i for i in S]
li_2 = list(set(li))
li_2.sort()
ascii_lowercase = "abcdefghijklmnopqrstuvwxyz"
ascii_lowercase_list = list(ascii_lowercase)
for k in range(0,len(S),1):
  if li_2[k] != ascii_lowercase_list[k]:
    print(ascii_lowercase_list[k])
    break
  elif li_2 == ascii_lowercase_list:
    print("None")
    break

Fixed submission:
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")

Buggy python submission (verdict: Runtime Error):
S = str(input())
li = [i for i in S]
li_2 = list(set(li))
li_2.sort()
ascii_lowercase = "abcdefghijklmnopqrstuvwxyz"
ascii_lowercase_list = list(ascii_lowercase)
for k in range(0,len(S),1):
  if li_2[k] != ascii_lowercase_list[k]:
    print(ascii_lowercase_list[k])
    break
  elif k == len(S)-1:
    print("None")
    break

Fixed submission:
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")

Buggy python submission (verdict: Runtime Error):
S = str(input())
li = [i for i in S]
li_2 = list(set(li))
li_2.sort()
ascii_lowercase = "abcdefghijklmnopqrstuvwxyz"
ascii_lowercase_list = list(ascii_lowercase)
for i in range(0,len(S),1):
  if li_2[i] != ascii_lowercase_list[i]:
    print(ascii_lowercase_list[i])
    break
  elif i+1 == len(S):
    print("None")
    break


Fixed submission:
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")