add up the cost of shopping

print("Hi, I am Marvin, your personal bot.")
command = input("How can I help? ")
if command == "make a shopping list":
    shopping = []

    R1 = input ( "How many items of shopping do you have? ")
    i = 0
    j = 0
    for item_number in range(int(R1)):
        item = input ( "What is the No." + str(item_number+1) + " item to buy today? " )
        shopping.append(item)
        i = i+1
        cost_str= input( " How much is this ?" )
        j = j + int(cost_str)



    count = 0
    print ("your shopping list items are :  " )
    for item in shopping:
        print(item)
        count = count + 1


print ("you have " + R1 + " items in your shopping list. Shopping will cost  " + str(j) + " RMB . " )