$\frac{1}{2}$ $\frac{1}{2}$

$1A2\sum _3\stackrel{\rightharpoonup }{4}||41-2$ $1A2\sum _3\stackrel{\rightharpoonup }{4}||41-2$

$1$ $1$

$''1''$ $''1''$

$\stackrel{2}{1}$ $\stackrel{2}{1}$

$\underset{2}{1}$ $\underset{2}{1}$

$\underset{2}{\overset{3}{1}}\underset{2}{\overset{3}{1}}$ $\underset{2}{\overset{3}{1}}\underset{2}{\overset{3}{1}}$