from tkinter import *

def f1():
    root.quit()

root = Tk()
root.geometry("400x400")
lab1 = Label(text="Добро пожаловать")
lab1.pack()


X = 0
Y = 0
lab_sum = Label(text = str(X) + "+" + str(Y))
lab_sum.pack()

but_exit = Button(text="Exit", command=f1)
but_exit.pack()
but_exit.place(x=350, y=350)

root.mainloop()