Row Reduction & Rank · Challenge problem

A $2 \times 4$ matrix $R$ is in reduced row echelon form with pivots in columns 1 and 2, and the solution set of $R\mathbf x = \mathbf 0$ is spanned by $(1, -1, 1, 0)$ and $(2, 0, -1, 1)$. What is the entry of $R$ in row 1, column 4?