function ruota(r,M,Nt)
plot(r(1,:),r(2,:))
    axis([-6 6 -6 6])
    axis('square')
pause
for k=1:Nt
    r = M*r;
    plot(r(1,:),r(2,:))
    axis([-6 6 -6 6])
    axis('square')
    drawnow
end

