from tkinter import *

def add_on():
    num = enter_txt.get()
    num = int(num)
    answer = output_txt["text"]
    answer = int(answer)
    total = num + answer
    output_txt["text"] = total

def reset():
    total = 0
    output_txt["text"] = 0
    enter_txt.delete(0, 'end')
    enter_txt.focus()

total = 0
num = 0

window = Tk()
window.title("Agregando números")
window.geometry("450x100")

enter_lbl = Label(text="Ingresa un número:")
enter_lbl.place(x=50, y=20, width=120, height=25)

enter_txt = Entry()
enter_txt.place(x=175, y=20, width=120, height=25)
enter_txt["justify"] = "center"
enter_txt.focus()

add_btn = Button(text="Agregar", command=add_on)
add_btn.place(x=300, y=20, width=55, height=25)

output_lbl = Label(text="TOTAL = ")
output_lbl.place(x=110, y=50, width=70, height=25)

output_txt = Message(text=0)
output_txt.place(x=175, y=50, width=120, height=25)
output_txt["bg"] = "white"
output_txt["relief"] = "sunken"

clear_btn = Button(text="Borrar", command=reset)
clear_btn.place(x=300, y=50, width=55, height=25)

window.mainloop()