The circuit as shown in figure, the equivalent gate is

Let the outputs of individual gates be labelled as X, Y and Z respectively starting from left to right. we have:

$$\begin{gathered} Z=\overline{Y} \\ Y=\overline{XX}=\overline{X}+\overline{X}=\overline{X} \\ X=\overline{A+B} \\ \Rightarrow Z=X=\overline{A+B} \end{gathered}$$.

Thus, the combination behaves as NOR gate.