private List<Vertex> MakeConvexHull()
        {
            int n = Vertices.Count;
            int k = 0;
            Vertices.Sort((left, right) => (left.X.ToString() + left.Y.ToString()).CompareTo(right.X.ToString() + right.Y.ToString()));
            List<Vertex> Hull = new List<Vertex>();
            for (int i = 0; i < n; Hull[k++] = Vertices[i++])
            {
                for (; k >= 2 && Cross(Hull[k - 2], Hull[k - 1], Vertices[i]) >= 0; --k)
                    ;
            }
            for (int i = n - 2, t = k; i >= 0; Hull[k++] = Vertices[i--])
            {
                for (; k > t && Cross(Hull[k - 2], Hull[k - 1], Vertices[i]) >= 0; --k)
                    ;
            }
            int m = 0;
            if (Hull[0].X == Hull[1].X && Hull[0].Y == Hull[1].Y)
                m = 1;
            for (int i = 0; i < n; i++)
            {
                if (i > k - 2 - m)
                    Hull[i].Should_be_deleted = true;
            }
            Hull.RemoveAll(i => i.Should_be_deleted == true);
            return Hull;
        }