Linear Cryptanalysis - Modern Cryptanalysis: Techniques for Advanced Code Breaking - page 144

Let's consider a simple example of using the piling-up lemma on the EASY1 cipher.
Although not shown in the Table 6-4 diagram, one of the equations is X 0 = Y 4 , which has a bias of -4. If you
look at the diagram of the EASY1 cipher in Figure 6-3 , there is one fairly easy pattern to follow using this bias:
In the second from the right S-box (second least significant), we have bit 4 of the S-box's output being mapped
to bit 0 of the same S-box's input for the next round.