Shortest missing

You are given a string of n characters from the range a–z.

Your task is to find out how long is the shortest string that does not appear as a contiguous substring in the given string.

You may assume that n is at most 10^5.

In a file nostring.py, implement a function find that returns the length of the missing string.

def find(s):
    # TODO

if __name__ == "__main__":
    print(find("zzz")) # 1
    print(find("aybabtu")) # 1
    print(find("abcdefghijklmnopqrstuvwxyz")) # 2