[Tkinter] Error in Tkinter

[moste]

First of all, thank you very match for your quick reply.
I used eval because i want to do some math after this and actually i didn't know about DoubleVar, as i said im new in tkinter.
Now i have a new problem.

y = DoubleVar()
z = DoubleVar()

entry_1 = Entry(root, textvariable=y)
entry_1.grid(row=0, column=1)
entry_2 = Entry(root, textvariable=z)
entry_2.grid(row=1, column=1)



y = (y.get())
z = (z.get())


x = z / (y + 1)
m = z - x 
k = ((z-x)/z)*100

i adjusted the code and now i got this error:
k = ((z-x)/z)*100
ZeroDivisionError: float division by zero

And the problem is that the error pops up before entering any data, just running the script.

And i also use python 3.6 :D