diff --git a/hackathon/objectdetection.py b/hackathon/objectdetection.py
index 76af579..5c23159 100644
--- a/hackathon/objectdetection.py
+++ b/hackathon/objectdetection.py
@@ -238,3 +238,43 @@ def evaluate(dataset, params, static, key, config, seed):
     logger.info("Evaluation completed!")
 
     return params
+
+def loss_fn(params, static, inputs, targets, num_classes, lambda_box=5.0, lambda_obj=1.0, lambda_cls=1.0):
+    """
+    params, static: from eqx.partition
+    inputs: batch of images [B, C, H, W]
+    targets: dict of ground truth values
+    """
+    model = eqx.combine(params, static)
+
+    
+    preds = jax.vmap(model)(inputs)  # output: [B, H, W, A * (5 + C)]
+
+    obj_mask = targets["object_mask"]
+    bbox_target = targets["bbox_target"]
+    class_target = targets["class_target"]
+
+    pred_bbox = preds[..., :4]
+    pred_obj = preds[..., 4:5]
+    pred_cls = preds[..., 5:]
+
+    
+    bbox_loss = jnp.abs(pred_bbox - bbox_target) * obj_mask
+    bbox_loss = jnp.sum(bbox_loss) / (jnp.sum(obj_mask) + 1e-6)
+
+    
+    obj_loss = optax.sigmoid_binary_cross_entropy(pred_obj, obj_mask)
+    obj_loss = jnp.mean(obj_loss)
+
+   
+    cls_loss = optax.sigmoid_binary_cross_entropy(pred_cls, class_target)
+    cls_loss = jnp.sum(cls_loss * obj_mask) / (jnp.sum(obj_mask) + 1e-6)
+
+    total_loss = lambda_box * bbox_loss + lambda_obj * obj_loss + lambda_cls * cls_loss
+
+    return total_loss, {
+        "total_loss": total_loss,
+        "bbox_loss": bbox_loss,
+        "obj_loss": obj_loss,
+        "cls_loss": cls_loss
+    }