\begin{center}
\begin{forest}
for tree={circle, draw,
            minimum size=2em,
            inner sep=0pt,
            s sep=2mm,
            l sep=1mm}   
	[$\lnot$, name=not_one
		[$\lor$, name=or
			[$a$, name=a]
			[$\lnot$, name=not_two
                [$\lnot$, name=not_three
           			[$b$, name=b]
                ]
			]
		]		
	]
\end{forest}
\end{center}

Every leaf is a atomic variable and the other nodes are labeled with logical operators, thus this is a well-formed formula.