Blackjack program with a problem with concationation

This is my blackjack python script. I was having an issue and I think it is a problem with the concatenation of the hit function. I have tried looking online and reading other questions but couldn't find one that applied. I am not sure if there are other problems with this script and if you find any please share. Thanks for any help if you can give any. This is my code:

import random
#run = True is a setup for a later use in a while true loop
run = True
#prints out a welcome
print("Welcome to blackjack in Python, hope you enjoy")
print("Side note: All Ace's are treated with a value of one")
#This generates all the cards and since their are 13 card not counting suites              
dealer_showing = random.randint(1, 13)
dealer_hiding = random.randint(1, 13)
player_1st_card = random.randint(1, 13)
player_2nd_card = random.randint(1, 13)
dealers_cards = dealer_hiding + dealer_showing
player_cards = player_1st_card + player_2nd_card
#Their is no card worth 11, 12, or 13 points in blackjack so this gives all "face cards" a value of 10
def check_13(who_to_check):
    if who_to_check >= 11:
        who_to_check = 10
        return who_to_check
#This uses the function seen up above
check_13(dealer_showing)
check_13(player_1st_card)
check_13(player_2nd_card)
check_13(dealer_hiding)
#This tells the user what their card value is and what the user is showing
print("Your card value is {}".format(player_cards))
print("The dealer is showing a {}".format(dealer_showing))

# Here I put in a function that checks if the player or computer have busted or won
def check_cards():
    if player_cards > 21 and dealers_cards > 21:
        print("Both you and the computer have busted")
        run = False
    elif player_cards > 21:
        print("I'm sorry the computer has won")
        run = False
    elif dealers_cards > 21:
        print("The computer has busted you win")
        run = False
    elif player_cards == 21 and dealers_cards == 21:
        print("Both you and the dealer have won")
    elif player_cards == 21:
        print("You have brought down the house")
    elif dealers_cards == 21:
        print("I'm sorry the computer has won")
    # Here I put in a function that hits
    def hit():
        player_cards = player_cards + check_13(random.randint(1, 10))
        return player_cards
        print("Your card value is now {}".format(player_cards))
    # Here I put in a function that lets the player stay
    def stay():
        if player_cards > dealers_cards:
            print("Congratulations you have won and beat the Computer")
            run = False
        elif player_cards == dealers_cards:
            print("You and the Computer have tied")
        else:
            print("I'm sorry the computer has won")
            False
    #Here is what the program runs over and over again in this loop
    while run == True:
        check_cards()
        print(player_cards)
        input_hit = input("Would you like to hit y/n:  ")

    if input_hit == "y":
        hit()
    elif input_hit == "n":
        stay()