$\underset{O}{O}\frac{1}{2}$ ${\displaystyle \underset{O}{O}\frac{1}{2}}$ $\begin{array}{c}\underset{O}{O}\frac{1}{2}\end{array}$ $O_O\frac{1}{2}$ $O_O\frac{1}{2}$ $\begin{array}{c}{O}_O\frac{1}{2}\end{array}$