Problem 3. (15 points) In parallelogram $A B C D$, side $A D$ is divided into equal parts by points $A_{1}, A_{2}, \ldots, A_{2020}$. Point $E_{1}$ is the intersection point of lines $B A_{1}$ and $A C$. Determine what fraction of diagonal $A C$ segment $A E_{1}$ constitutes.