Plotting 4 vortices in Matlab

I have been set a question where I have to plot the trajectories of 4 potential vortices as they induce motion in each other. I know that the motion of each vortex should be relative to the other 3, and as such dx/dt and dy/dt for each vortex should be a sum of the other three but i dont know how to do this on matlba. the motion is

dx_j/dt=-1/2*pi * sum(gamma*(y_j-y_i)/(r_ij^2) dy_j/dt=1/2*pi * sum(gamma*(x_j-x_i)/(r_ij^2)

the sum is from i=1 to N and i cannot equal j.

Gamma is the circulation. Thanks!