import flet as ft

def main(page: ft.Page):
    page.title = "Döner Rechner"
    def d_to_g(e):
        tgeld.value = str(int(tdoner.value)*int(tpreis.value))
        page.update()
    def g_to_d(e):
        tdoner.value = str(int(tgeld.value)/int(tpreis.value))
        page.update()

    tpreis = ft.TextField(value=5,keyboard_type="NUMBER",scale=0.5)
    tdoner = ft.TextField(on_change=d_to_g,keyboard_type="NUMBER")
    tgeld = ft.TextField(on_change=g_to_d,keyboard_type="NUMBER")
    page.add(ft.SafeArea(ft.Row([tpreis,ft.Text("Preis")])),
             ft.Row([tdoner,ft.Text("Döner")]),
             ft.Text("zu"),
             ft.Row([tgeld,ft.Text("Euro")]))

ft.app(main)