Consider a simple substitution$-$permutation network with the following S$-$box and permutation.

$($a$)$ Let $U_1,U_2,U_3$ be the input bits and $V_1,V_2,V_3$ the output bits of the S$-$box. Calculate the bias $U_1+V_3.$

$($b$)$ Note that this S$-$box is not random: the bits are simply shifted left by one, wrapping around. This means that the S$-$box will affect the trail in the same way. Using this information, draw the trail of the the indicated bit through the following network. $($The round keys here are fixed and we are not trailing their effects.