Solution to Question 30 on Linear Algebra in GATE 2024 examination

Given a real subspace W of R4, let W⊥ denote its orthogonal complement with respect
to the standard inner product on R4. Let W1 = Span{(1, 0, 0,−1)} and W2 =
Span{(2, 1, 0,−1)} be real subspaces of R4.
The dimension of W⊥1 ∩ W⊥ over R is
equal to (answer in integer)