Find the equivalent resistance across $A B$

$R_1$ and $R_2$ are in parallel order, so their equivalent resistance $R^{\prime}=\frac{2 \times 2}{2+2}=1 \Omega$
Similarly, equivalent resistance of $R_3$ and $R_4$ $R^{\prime \prime}=1 \Omega$
$R^{\prime}$ and $R^{\prime \prime}$ are in series order, so their equivalent resistance $\begin{aligned} R^{\prime \prime \prime} & =R^{\prime}+R^{\prime \prime} \\ & =1+1=2 \Omega \end{aligned}$ $R^{\prime \prime \prime}$ and $R_5$ are in parallel. So, $\quad R_{A B}=\frac{2 \times 2}{2+2}=1 \Omega$