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.
import string
t = list(string.ascii_lowercase)
s = input()

def find(s):
    for i in range(len(t)):
        if s.count(t[i]) == 0:
            return t[i]
    return 'None'

find(s)
import string
t = list(string.ascii_lowercase)
s = input()

def find(s):
    for i in t:
        if s.count(i) == 0:
            return i
    return 'None'

print(find(s))