[Tkinter] Error in Tkinter

[moste]

Hello guys, i am trying to create a programm with tkinter (for first time) and i got the follow error in one part of my code:

y = StringVar()
z = StringVar()

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 = str(y.get())
z = str(z.get())
y1=eval(y)
z1=eval(z)

When i run this loop i get the error "SyntaxError: unexpected EOF while parsing".
If i use float instead of eval i get another error "ValueError: could not convert string to float:"

Is there anyone who can help me?