# leastFrequentLetters(s) [20 pts]
# Write the function leastFrequentLetters(s), that takes a string s, and ignoring case (so "A" and "a" are treated 
# the same), returns a lowercase string containing the least-frequent alphabetic letters that occur in s, each 
# included only once in the result and then in alphabetic order. So:
# leastFrequentLetters("aDq efQ? FB'daf!!!") 
# returns "be". Note that digits, punctuation, and whitespace are not letters! Also note that seeing as we have not 
# yet covered lists, sets, maps, or efficiency, you are not expected to write the most efficient solution. Finally, 
# if s does not contain any alphabetic characters, the result should be the empty string ("")

def leastfrequentletters(s):
    k=s.lower()
    h=""
    p=""
    for i in k:
        if (i.isalnum()):
            h=h+i
    
    for j in h:
        if (h.count(j)==1):
            p=p+j
    
    return ("".join(sorted(p)))
