diff --git a/src/service/dialog_state_monitor.py b/src/service/dialog_state_monitor.py
index abebbea..7827de7 100644
--- a/src/service/dialog_state_monitor.py
+++ b/src/service/dialog_state_monitor.py
@@ -1,5 +1,6 @@
 from src.model.frame import Frame
 import copy
+import json
 
 
 def normalize(value):
@@ -8,7 +9,9 @@ def normalize(value):
 
 
 class DialogStateMonitor:
-    def __init__(self):
+    def __init__(self, initial_state_file: str = '../attributes.json'):
+        with open(initial_state_file) as file:
+            constants = json.load(file)
         self.__initial_state = dict(belief_state={
             'order': [],
             'address': {},
@@ -19,17 +22,20 @@ class DialogStateMonitor:
             'time': {},
             'name': {},
         },
+            total_cost=0,
             stages=[
                 {'completed': False, 'name': 'collect_food'},
                 {'completed': False, 'name': 'collect_drinks'},
                 {'completed': False, 'name': 'collect_address'},
             ],
+            was_previous_order_invalid=False,
             constraints={
                 'order': [
                     'sauce',
                     'pizza',
                 ],
             },
+            constants=constants,
             history=[])
         self.state = copy.deepcopy(self.__initial_state)
 
@@ -55,6 +61,12 @@ class DialogStateMonitor:
                 stage['completed'] = True
                 return
 
+    def pizza_exists(self, name: str) -> bool:
+        return normalize(name) in self.state['constants']['pizza']
+
+    def get_total_cost(self) -> int:
+        return self.state['total_cost']
+
     def update(self, frame: Frame) -> None:
         self.state['history'].append(frame)
         if frame.source != 'user':
@@ -62,7 +74,14 @@ class DialogStateMonitor:
         if frame.act == 'inform/order':
             new_order = dict()
             for slot in frame.slots:
-                new_order[slot.name] = normalize(slot.value)
+                value = normalize(slot.value)
+                if slot.name == 'pizza':
+                    if self.pizza_exists(value) is False:
+                        self.state['was_previous_order_invalid'] = True
+                        return
+                    self.state['was_previous_order_invalid'] = False
+                    self.state['total_cost'] += self.state['constants']['pizza'][value]['price']
+                new_order[slot.name] = value
             self.state['belief_state']['order'].append(new_order)
         elif frame.act == 'inform/address':
             for slot in frame.slots:
diff --git a/src/test/dialog_state_monitor.py b/src/test/dialog_state_monitor.py
index f6509d4..363e302 100644
--- a/src/test/dialog_state_monitor.py
+++ b/src/test/dialog_state_monitor.py
@@ -4,22 +4,28 @@ from src.model.slot import Slot
 
 dsm = DialogStateMonitor()
 
-frame1 = Frame('user', 'inform/order', [Slot('pizza', 'margaritta'), Slot('sauce', 'ketchup')])
+assert dsm.pizza_exists('capri') is True
+assert dsm.state['was_previous_order_invalid'] is False
+
+frame1 = Frame('user', 'inform/order', [Slot('pizza', 'margarita'), Slot('sauce', 'ketchup')])
 dsm.update(frame1)
 assert dsm.get_last_order_missing_fields() == []
-frame2 = Frame('user', 'inform/order', [Slot('pizza', 'carbonara')])
+assert dsm.get_total_cost() == 20
+frame2 = Frame('user', 'inform/order', [Slot('pizza', 'tuna')])
 dsm.update(frame2)
 assert dsm.get_last_order_missing_fields() == ['sauce']
+assert dsm.get_total_cost() == 60
 frame3 = Frame('user', 'inform/order-complete', [])
 dsm.update(frame3)
 
-assert dsm.state['belief_state']['order'][0]['pizza'] == 'margaritta'
+assert dsm.state['belief_state']['order'][0]['pizza'] == 'margarita'
 assert dsm.state['belief_state']['order'][0]['sauce'] == 'ketchup'
 assert dsm.state['belief_state']['order-complete'] is True
 assert dsm.state['history'][0] == frame1
 assert dsm.state['history'][1] == frame2
 assert dsm.state['history'][2] == frame3
 
+
 assert dsm.get_current_active_stage() == 'collect_food'
 dsm.mark_current_stage_completed()
 assert dsm.get_current_active_stage() == 'collect_drinks'
@@ -30,7 +36,16 @@ assert dsm.get_current_active_stage() is None
 
 dsm.reset()
 
+assert dsm.get_total_cost() == 0
 assert dsm.get_current_active_stage() == 'collect_food'
 assert dsm.state['belief_state']['order'] == []
 assert dsm.state['belief_state']['order-complete'] is False
 assert len(dsm.state['history']) == 0
+
+dsm.reset()
+
+frame1 = Frame('user', 'inform/order', [Slot('pizza', 'buraczna')])
+dsm.update(frame1)
+assert dsm.state['was_previous_order_invalid'] is True
+assert dsm.state['belief_state']['order'] == []
+assert dsm.get_total_cost() == 0