A force $F = - K\left( {y\widehat i + x\widehat j} \right) (where K is a positive constant) acts on a particle moving in the xy plane. Starting from the origin, the particle is taken along the positive x axis to the point \left( {a,0} \right), and then parallel to the y axis to the point \left( {a,a} \right)$. The total work done by the force F on the particle is