На сторонах \(\displaystyle AB{\small,}\) \(\displaystyle BC{\small,}\) \(\displaystyle CD\) и \(\displaystyle AD\) квадрата \(\displaystyle ABCD\) отметили соответственно точки \(\displaystyle R{\small,}\) \(\displaystyle M{\small,}\) \(\displaystyle F\) и \(\displaystyle N\) так, что \(\displaystyle MN \perp RF{\small.}\) Найдите длину отрезка \(\displaystyle RF{\small,}\) если \(\displaystyle AB=4{\small,}\) \(\displaystyle MN=5{\small.}\)

\(\displaystyle RF=\) [ ]