At A current is distributed and at B currents are collected. Between A and B, the distribution is symmetrical. It has been shown in the figure. It appears that current in AO and OB remains same. At O, current returns back without any change. If we detach O from AB there will not be any change in distribution. Now, CO & OD will be in series hence its total resistance
It is in parallel with CD, so, equivalent resistance

This equivalent resistance is in series with AC & DB so, total resistance

Now is parallel to AB, that is, , so total resistance

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