public class Vector2D {
    double x, y;

    public Vector2D() {
        x = 0;
        y = 0;
    }

    public Vector2D(double x, double y) {
        this.x = x;
        this.y = y;
    }

    public Vector2D add(Vector2D v) {
        return new Vector2D(v.x + x, v.y + y);
    }

    public void add2(Vector2D v) {
        x += v.x;
        y += v.y;
    }

    public Vector2D sub(Vector2D v) {
        return new Vector2D(x - v.x, y - v.y);
    }

    public void sub2(Vector2D v) {
        x -= v.x;
        y -= v.y;
    }

    public Vector2D mult(double a) {
        return new Vector2D(x * a, y * a);
    }

    public void mult2(double a) {
        x *= a;
        y *= a;
    }

    public String toString() {
        return "(" + x + ";" + y + ")";
    }

    public double length() {
        return Math.sqrt(x * x + y * y);
    }

    public double scalarProduct(Vector2D v) {
        return x * v.x + y * v.y;
    }

    public double cos(Vector2D v) {
        return scalarProduct(v) / (length() * v.length());
    }

    public boolean equals(Vector2D v) {
        return v.x == x && v.y == y;
    }
}