def cover_the_gap_students(shed, n, group):
    pr = [-1] * 6
    for lesson in range(7):
        for day in range(6):
            if shed[group][lesson][day] != 0:
                pr[day] = lesson
    cur = []
    for i in range(len(pr)):
        if pr != -1:
            cur.append(i)
    rand = choice(len(cur))
    rs = cur[rand]
    first = -1
    # here we are a little bit greedy
    for lesson in range(7):
        if shed[group][lesson][rand] != 0 and first == -1:
            first = lesson
        if shed[group][lesson][rand] != 0 and first != -1  and lesson < rs:
            pos.append(lesson)
    # if possible returns shed, else returns -1
    for i in range(len(pos)):
        shed[group][rs][rand], shed[group][pos[i]][rand] = shed[group][pos[i]][rand],shed[group][rs][rand]
        if checkcorrectness(shed,n,a):
            return shed
        shed[group][rs][rand], shed[group][pos[i]][rand] = shed[group][pos[i]][rand],shed[group][rs][rand]
    return -1







def cover_the_gap_teachers():
    pass