Dimension of an orthogonal complement

Consider the subspace $U = \{(x_1, x_2, x_3, x_4) \in \mathbb{R}^4 : x_1 + x_2 + x_4 = 0, \; x_3 = 0\}$. Let $U^{\perp}$ be its orthogonal complement. The value of $\dim(U^{\perp})$ equals ________ (answer in integer).