UnboundLocalError: local variable 'tickets_remaining' referenced before assignment

TICKET_PRICE = 10
tickets_remaining = 100  

#run this program until tickets finished.

def runProgram():
    #Output how many tickets are remaining using the tickets_remaining variable
    print(("There are {} tickets remaining, be sure to grab it quick!!!").format(tickets_remaining))

    #Gather the users' name and assign it to a new variable.
    user_name = input("What is your name?   ")

    #Prompt the users by name and ask the user how many tickets they want
    ticket_number = int(input("Welcome to the concert, {}. How many tickets do you want to purchase?   ".format(user_name)))

    if ticket_number < tickets_remaining:
        #Calculate the price (number of tickets x ticket price) and assign thhat to a variable
        total_price = TICKET_PRICE * ticket_number

        #output the price to the screen
        print('You have ordered {} tickets. The total price is ${}'.format(ticket_number,total_price))

        #prompt users if they want to proceed? Y/N
        user_proceed = input('Do you want to proceed to the payment process? Y or N   ')
        user_proceed = user_proceed.upper()

        #If they want to proceed, 
        if user_proceed == "Y":
            #we output it on the screen "sold" to confirm purchase. 
            #Gather credit card information and process it.
            print('SOLD!!!')

            #and then decrement the ticket remaining.
            tickets_remaining -= ticket_number

        #otherwise just thank them
        else:
            print('Thank you, {}'.format(user_name))
    else:
        print('We are sorry, there are only {} tickets available.')

if tickets_remaining > 0:
    runProgram()
else:
    print("We have sold out all the tickets")