TypeError: unsupported operand type(s) for -: 'str' and 'str'

Написал функцию DrawLine в tkinter, но при её использовании появляется ошибка:

TypeError: unsupported operand type(s) for -: 'str' and 'str'

С чем она может быть связана?

Код:

import matplotlib
import numpy as np
import matplotlib.pyplot as plt
from tkinter import *

window = Tk()

img=np.ones((1000,1000,3))


def DrawLine(x1, y1, x2, y2):
    dx=abs(x2-x1) 
    dy=abs(y2-y1)
    
    if x1<x2:
        xs=1
    else:
        xs=-1
        
    if y1<y2:
        ys=1
    else:
        ys=-1
        
    x=x1
    y=y1
    
    if dx>dy:
        p=2*y-dx
        
        while x!=x2:
            x=x+xs
            if p>0:
                y=y+ys
                p=p+2*dy-2*dx
                
            else:
                p=p+2*dy
                
            img[y,x]=0
      
    else:
        p=2*x-dy
        
        while y!=y2:
            y=y+ys
            if p>0:
                x=x+xs
                p=p+2*dx-2*dy
                
            else:
                p=p+2*dx
                
            img[y,x]=0


def clicked():
    a = txt.get()
    b = txt2.get()
    c = txt3.get()
    d = txt4.get()
    plt.rcParams["figure.figsize"]=(5,5)
    plt.rcParams["figure.dpi"]=80
    DrawLine(a,b,c,d)
    plt.imshow(img)
    plt.show()


window.title("Programma")
window.geometry('350x300+600+200')
txt = Entry(window,width=30)
txt.grid(column = 2, row = 0)
lbl = Label(window, text = "Введите координату x1")
lbl.grid(column = 1, row = 0)
txt2 = Entry(window,width=30)
txt2.grid(column = 2, row = 1)
lbl2 = Label(window, text = "Введите координату y1")
lbl2.grid(column = 1, row = 1)
txt3 = Entry(window,width=30)
txt3.grid(column = 2, row = 2)
lbl3 = Label(window, text = "Введите координату x2")
lbl3.grid(column = 1, row = 2)
txt4 = Entry(window,width=30)
txt4.grid(column = 2, row = 3)
lbl4 = Label(window, text = "Введите координату y1")
lbl4.grid(column = 1, row = 3)
btn = Button(window, text = "Нажми на меня", command=clicked)
btn.grid(column = 1, row = 4)
window.mainloop()