Continuously update string in python curses

Trying to learn curses, I wrote this script that lets the user input two numbers, then outputs their sum and their difference:

import curses

screen = curses.initscr()
screen.refresh()

height = 4
width = 25
abwindow =  curses.newwin(height, width, int(0.15*int(curses.LINES)), int(curses.COLS/2) - width)
abwindow.addstr(1, 2, "a is : ")
abwindow.addstr(2, 2, "b is : ")

abwindow.border()
abwindow.refresh()

sumdiffwindow = curses.newwin(height, width, int(0.15*int(curses.LINES)), int(curses.COLS/2))
sumdiffwindow.addstr(1, 2, "a + b is : ")
sumdiffwindow.addstr(2, 2, "a - b is : ")

sumdiffwindow.border()
sumdiffwindow.refresh()

atocheck = abwindow.getstr(1, 10, 7)
btocheck = abwindow.getstr(2, 10, 7)

try:
    a = float(atocheck)
    b = float(btocheck)
    sum = a + b
    diff = a - b
    sumdiffwindow.addstr(1, 14, "%g" %(sum))
    sumdiffwindow.addstr(2, 14, "%g" %(diff))
except ValueError:
    sumdiffwindow.addstr(1, 14, "nan")
    sumdiffwindow.addstr(2, 14, "nan")

sumdiffwindow.refresh()
curses.curs_set(0)

while True:
    curses.noecho()
    c = screen.getch(1, 1)
    if c == ord('q') or c == ord('Q'):
        break

curses.endwin()

Once the two numbers are inputed, (if they are numbers) it calculates the sum and difference, and then it idles until the user presses 'q' to go back to the terminal. How can I change it so that I can update a and b as much as I want (using the keyboard up and down arrows to navigate between the two input boxes), while continuously displaying their current sum and difference?

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