diff --git a/exercise4/wave_2d_parallel.c b/exercise4/wave_2d_parallel.c
index 0b14a81..6e3442e 100644
--- a/exercise4/wave_2d_parallel.c
+++ b/exercise4/wave_2d_parallel.c
@@ -114,23 +114,28 @@ void domain_save(int_t step) {
 // Neumann (reflective) boundary condition
 void boundary_condition(void) {
   // BEGIN: T7
-  // Vertical ghost layers (bottom and top)
-  for (int_t i = 0; i < M; i++) {
-    U(i, -1) = U(i, 1);    // bottom ghost row <- mirror of row 1
-    U(i, N) = U(i, N - 2); // top ghost row <- mirror of row N-2
-  }
 
-  // Horizontal ghost layers (left and right)
-  for (int_t j = 0; j < N; j++) {
-    U(-1, j) = U(1, j);    // left ghost col <- mirror of col 1
-    U(M, j) = U(M - 2, j); // right ghost col <- mirror of col M-2
+  if (up == MPI_PROC_NULL) {
+    // note: -1 to handle corners automatically
+    for (int_t j = -1; j < local_N + 1; j++) {
+      U(-1, j) = U(1, j);
+    }
+  }
+  if (down == MPI_PROC_NULL) {
+    for (int_t j = -1; j < local_N + 1; j++) {
+      U(local_M, j) = U(local_M - 2, j);
+    }
+  }
+  if (left == MPI_PROC_NULL) {
+    for (int_t i = -1; i < local_M + 1; i++) {
+      U(i, -1) = U(i, 1);
+    }
+  }
+  if (right == MPI_PROC_NULL) {
+    for (int_t i = -1; i < local_M + 1; i++) {
+      U(i, local_N) = U(i, local_N - 2);
+    }
   }
-
-  // Corner ghost cells (use ghost indices)
-  U(-1, -1) = U(1, 1);       // bottom-left
-  U(-1, N) = U(1, N - 2);    // top-left
-  U(M, -1) = U(M - 2, 1);    // bottom-right
-  U(M, N) = U(M - 2, N - 2); // top-right
   // END: T7
 }