The picture gives the aggregate plan of Wonderful tablet produced by a Swedish firm. It is known that the firm uses a level plan with backorders. Given this information, what is the number in the cell with "xx"

\begin{tabular}{|ccccccc|} \hline Month\# Forecast\# Starting inv.\# Net requirement\# Prod. rate\# Ending inv.\# Stockout \\ Jan & 1200 & 321 & 879 & 1112 & 233 & 0 \\ Feb & 2000 & 233 & 1767 & 1112 & 0 & 655 \\ Mar & 1000 & 0 & 1655 & 1112 & 0 & 543 \\ Apr & 180 & 0 & xx & 1112 & 389 & 0 \\ May & 1500 & 389 & 1111 & 1112 & 1 & 0 \\ \hline \end{tabular}