  With Skipper-CCD detectors it is possible to take multiple samples of the
charge packet collected on each pixel. After averaging the samples, the noise
can be extremely reduced allowing the exact counting of electrons per pixel. In
this work we present an analog circuit that, with a minimum number of
components, applies a double slope integration (DSI), and at the same time, it
averages the multiple samples producing at its output the pixel value with
sub-electron noise. For this prupose, we introduce the technique of using the
DSI integrator capacitor to add the skipper samples. An experimental
verification using discrete components is presented, together with an analysis
of its noise sources and limitations. After averaging 400 samples it was
possible reach a readout noise of 0.2\,$e^-_{RMS}/pix$, comparable to other
available readout systems. Due to its simplicity and significant reduction of
the sampling requirements, this circuit technique is of particular interest in
particle experiments and cameras with a high density of Skipper-CCDs.
