
If the 4 input bits at the upper left side are the same as the 4 input bits at the lower left side, then the output E is 1 (green) else the output of E is 0 (red).
If the 4 input bits at the upper left side are the same as the 4 input bits at the lower left side, then the output E is 1 (green) else the output of E is 0 (red).