diff --git a/LemmingSim.html b/LemmingSim.html
index a297aed..578c4e8 100644
--- a/LemmingSim.html
+++ b/LemmingSim.html
@@ -53,6 +53,7 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 <tr><td><label for="ffwdtimes">nr of experiments</label></td><td><input id="ffwdtimes" type="number" value="10" min="1" max="20" /></td></tr>
 <tr><td><label for="wanderRate">robot wander rate</label></td><td><input id="wanderRate" type="number" value="0.0002" step="0.0001"></td></tr>
 <tr><td><label for="boxFric">box friction</label></td><td><input id="boxFric" type="number" value="0.001" step="0.0005" min=0></td></tr>
+<tr><td><label for="boxX">Boxes:</label></td><td><input id="boxX" type="number" value="5" min=1 max=10>x<input id="boxY" type="number" value="5" min=1 max=10></td></tr>
 <tr><td><input type="button" value="Restart Slow" onclick="initSimulation(); sim.start()" /><input type="button" value="toggle pause" onclick="sim.toggle()"></td><td><input type="button" value="run experiments" onclick="runExperiments()" /><input type="button" value="clear" onclick="resultsTable.clear()" /></td></tr>
 <tr><td></td></tr>
       </tbody></table>
diff --git a/lemming_sim_student.js b/lemming_sim_student.js
index fe2d0ab..6fb6a52 100644
--- a/lemming_sim_student.js
+++ b/lemming_sim_student.js
@@ -170,10 +170,11 @@ class Simulation {
                 color
             });
         }
+        const [nBoxX, nBoxY] = ["boxX", "boxY"].map(x => +document.getElementById(x).value)
 
-        const startX = 100, startY = 100,
-              nBoxX = 5, nBoxY = 5,
-              gapX = 40, gapY = 30,
+        const gapX = 40, gapY = 30
+        const startX = (width - (simInfo.boxSize * nBoxX + (nBoxX-1) * gapX)) / 2,
+              startY = (height - (simInfo.boxSize * nBoxY + (nBoxY-1) * gapY)) / 2,
               stack = Matter.Composites.stack(startX, startY,
                                               nBoxX, nBoxY,
                                               gapX, gapY, getBox);