void Sq() {
        long long sq = 0;
        Point strt(0, 0);
        for (int i = 0; i < pnum - 1; i++) {
            Vector p1(strt, p[i]);
            Vector p2(strt, p[i + 1]);
            sq += (p1 ^ p2);
        }
        Vector p1(strt, p[pnum - 1]);
        Vector p2(strt, p[0]);
        sq += (p1 ^ p2);
        if (sq % 2 == 1 || sq % 2 == -1) {
            cout << abs(sq / 2) << ".5";
        } else {
            cout << abs(sq / 2);
        }
    }