The work done on a particle of mass m by a force, K $[\frac{\mathrm{x}}{{({\mathrm{x}}^2+{\mathrm{y}}^2)}^{3/2}}\hat{\mathrm{i}}+\frac{\mathrm{y}}{{({\mathrm{x}}^2+{\mathrm{y}}^2)}^{3/2}}\hat{\mathrm{j}}]$  (K being a constant of appropriate dimensions), when the particle is taken from the point (a, 0), to the point (0, a) along a circular path of radius a about the origin in the x-y plane is