from tkinter import *

def add_function():
    e3.insert(0, str(int(e1.get())+int(e2.get())))

def sub_function():
    e4.insert(0, str(int(e1.get())-int(e2.get())))

def clear_entries():
    e1.delete(0,10)
    e2.delete(0,10)
    e3.delete(0,10)
    e4.delete(0,10)

My_window = Tk()
My_window.title("My Simple Calculator")
My_window.geometry("400x400")

l1 = Label(My_window, text='1ος αριθμός:')
l1.grid(row=0, sticky=W)
e1 = Entry(My_window)
e1.grid(row=0, column=1)

l2 = Label(My_window, text='2ος αριθμός:')
l2.grid(row=1, sticky=W)
e2 = Entry(My_window)
e2.grid(row=1, column=1)

b1 = Button(My_window, text='Πρόσθεση', fg='navy', command=add_function)
b1.grid(row=2, sticky=W)
e3 = Entry(My_window)
e3.grid(row=2, column=1)

b2 = Button(My_window, text='Αφαίρεση', fg='navy', command=sub_function)
b2.grid(row=3, sticky=W)
e4 = Entry(My_window)
e4.grid(row=3, column=1)

b3 = Button(My_window, text='Καθαρισμός', fg='navy', command=clear_entries)
b3.grid(row=4, sticky=W)

My_window.mainloop()