def recursive_len(xs):
    if xs != []:
        return(1+recursive_len(xs[1:]))
    else:
        return(0)

def remove_duplicates(msg):
    if len(msg) <= 1:
        return(msg)
    elif msg[0] == msg[1]:
        return(remove_duplicates(msg[1:]))
    else:
        return(msg[0]+remove_duplicates(msg[1:]))

def binary(n):
    result = ""
    if n % 2 == 0:
        result = "0"
    else:
        result = "1"
    if (n == 0) or (n == 1):
        return(result)
    return binary(n//2) + result