def foo(l):
    if len(l) == 0:
        return None

    if len(l) == 1:
        return str(l[0])

    l = sorted(l)

    res = ''
    asc = False # ascending

    for i in range(1, len(l)):
        if l[i] == l[i-1]+1: # if ascending
            if not asc:      # place first and skip ascending elements
                asc = True
                res += str(l[i-1])+'-'
        else: # if next is not ascending,
            res += str(l[i-1])+','
            asc = False

    res += str(l[i]) # append last digit

    return res

if __name__ == "__main__":
    l = [10, 7, 8, 9, 1, 2, 3, 5, 99]
    print(foo(l))