Output at NOR gate $Y_1=\overline{A+B}$

The output of first NOR gate is input to the second NOR gate, so the output of second NOR gate is $Y_2=\overline{Y_1+Y_1}=\overline{Y_1}=\overline{\overline{A+B}}=A+B$

The output of NOT gate is $Y=\overline{Y_2}=\overline{A+B}$

The given electrical network becomes equivalent to the NOR gate.