Problem 19 (2018 AMC 8)

In a sign pyramid a cell gets a "$+" if the two cells below it have the same sign, and it gets a "$-$" if the two cells below it have different signs. The diagram below illustrates a sign pyramid with four levels. How many possible ways are there to fill the four cells in the bottom row to produce a "$+" at the top of the pyramid? [See figure in original PDF]