it doesnt work

gistfile1.txt

def script():
    print ("Type any city.")
    city = input()
    if city < "Denver":
        print ("This city comes before Denver alphabetically.")
    elif city > "Denver":
        print ("This city comes after Denver alphabetically.")
    elif city == "Denver":
        print ("You wrote Denver.")
        restart = raw_input("Would you like to restart?")
    if restart == "yes" or restart == "y":
        script()
    if restart == "no" or restart == "n":
        print ("Thanks for using my program.")
script()