# Enter your code here. Read input from STDIN. Print output to STDOUT
from collections import defaultdict
class SupermarketCheckout:
    checkout_line = defaultdict(list)

    def on_customer_enter(self, customer_id, line_number, num_items):
        self.checkout_line[customer_id].append(line_number)
        self.checkout_line[customer_id].append(num_items)
        pass

    def on_basket_change(self, customer_id, new_num_items):
        if customer_id in self.checkout_line:
            self.checkout_line[customer_id][1] = new_num_items
        pass

    def on_line_service(self, line_number, num_processed_items):

        for cust in self.checkout_line:
            lin = self.checkout_line[cust][0]

            if lin != line_number:
                continue

            items = self.checkout_line[cust][1]
            val = max(items - num_processed_items,0)
            self.checkout_line[cust][1] = val
            if val == 0:
                self.on_customer_exit(cust)


    def on_lines_service(self):
        for cust in self.checkout_line:
            items = self.checkout_line[cust][1]
            if items > 0:
                items -= 1
            self.checkout_line[cust][1] = items
            if items == 0:
                self.on_customer_exit(cust)

    def on_customer_exit(self, customer_id):
        # Don't change this implementation.
        print(customer_id)


smc = SupermarketCheckout()
smc.on_customer_enter(123,1,5)
smc.on_customer_enter(2,2,3)
smc.on_lines_service()
smc.on_customer_enter(3,1,2)
smc.on_line_service(1,6)