diff --git a/app.py b/app.py
index a2d2561..0c4cac1 100644
--- a/app.py
+++ b/app.py
@@ -62,6 +62,20 @@ def delete_item(item_id):
     return "", 204
 
 
+@app.route("/add", methods=["POST"])
+def add():
+    data = request.get_json()
+    if not data or "a" not in data or "b" not in data:
+        return jsonify(error="both 'a' and 'b' are required"), 400
+
+    try:
+        result = data["a"] + data["b"]
+    except TypeError:
+        return jsonify(error="'a' and 'b' must be numbers"), 400
+
+    return jsonify(result=result), 201
+
+
 _items_store = []
 
 
diff --git a/test_app.py b/test_app.py
index d5ae8f6..3044652 100644
--- a/test_app.py
+++ b/test_app.py
@@ -63,3 +63,23 @@ def test_delete_item(client):
 def test_delete_item_not_found(client):
     resp = client.delete("/items/999")
     assert resp.status_code == 404
+
+
+def test_add(client):
+    resp = client.post("/add", json={"a": 2, "b": 3})
+    assert resp.status_code == 201
+    data = resp.get_json()
+    assert data["result"] == 5
+
+
+def test_add_missing_fields(client):
+    resp = client.post("/add", json={"a": 2})
+    assert resp.status_code == 400
+
+    resp = client.post("/add", json={})
+    assert resp.status_code == 400
+
+
+def test_add_invalid_types(client):
+    resp = client.post("/add", json={"a": "x", "b": 3})
+    assert resp.status_code == 400