2015 CIMC keystage3-IWYMIC individual Question 8

The three two-digit numbers $\overline{ab}$, $\overline{cd}$ and $\overline{ad}$ are such that $(\overline{ab})^2+(\overline{cd})^2=(\overline{ad})^2$. Find the minimum value of the four-digit number $\overline{abcd}$.