def add_student(dic, name, surname1, surname2, lsub):
    dic[(name, surname1, surname2)] = lsub

def subject_student(dic, name, surname1, surname2):
    if (name, surname1, surname2) not in dic:
        return []
    else:
        return dic[(name, surname1, surname2)]

def listed(dic,subj):
    l = []
    for st in dic:
        if subj in dic[st]:
            namest = st[1] + ' ' + st[2] + ', ' + st[0]
            l.append(namest)
    l.sort()
    return l

def students_per_subject(dic):
    dsubj = {}
    for st in dic:
        lsubj = dic[st]
        for subj in lsubj:
            if subj in dsubj:
                dsubj[subj] = dsubj[subj] + 1
            else:
                dsubj[subj] = 1
    return dsubj