t = 0;

while (t < 1000) 

  r = sqrt(x*x + y*y);
  a = - G*M*m / (r*r);
  ax = a*(x/r);
  ay = a*(y/r);

  vx = vx + ax*dt;
  vy = vy + ay*dt;

  x = x + vx*dt;
  y = y + vy*dt;

  t = t + dt;

end