Place two blocks next to each other (distance: one block). Install two redstone-lanes (both connected to a lever) which end in these blocks.
Put two redstone-torches on the other side of these blocks and extract the signal of the torches by connecting the redstone-lanes.
Build another block right next to the extracted signal, and put a redstone torch at its back, too.
The output-signal becomes positive if one or both inputs are negative (NOT-AND=Not both inputs are positive).