calculator in progress

from tkinter import *

root = Tk()
root.title("Calculator")
root.geometry("220x550")

def plus_button_function():
    global plus_first
    plus_first = int(e.get())
    e.delete(0, END)

def equal_button_function():
    global plus_second
    plus_second = int(e.get())
    plus_sum = plus_first + plus_second
    e.insert(0, plus_sum)

def b1_function():
    e.insert(0, "1")

def b2_function():
    e.insert(0, "2")

def b3_function():
    e.insert(0, "3")

def b4_function():
    e.insert(0, "4")

def b5_function():
    e.insert(0, "5")

def b6_function():
    e.insert(0, "6")

def b7_function():
    e.insert(0, "7")

def b8_function():
    e.insert(0, "8")

def b9_function():
    e.insert(0, "9")

e = Entry(root)
equal_button = Button(root, text = "=", command = equal_button_function)

b1 = Button(root, text = "1", command = b1_function)
b2 = Button(root, text = "2", command = b2_function)
b3 = Button(root, text = "3", command = b3_function)
b4 = Button(root, text = "4", command = b4_function)
b5 = Button(root, text = "5", command = b5_function)
b6 = Button(root, text = "6", command = b6_function)
b7 = Button(root, text = "7", command = b7_function)
b8 = Button(root, text = "8", command = b8_function)
b9 = Button(root, text = "9", command = b9_function)

plus_button = Button(root, text = "+", command = plus_button_function)

e.grid(row = 0, column = 0)
equal_button.grid(row = 0, column = 1)
b1.grid(row = 1, column = 1)

root.mainloop()