7. Given a four-digit number $\overline{A B C D}$ that satisfies the following property: $\overline{A B} 、 \overline{B C} 、 \overline{C D}$ are all perfect squares (a perfect square is a number that can be expressed as the square of an integer, for example, $4=2^{2}, 81=9^{2}$, so we call $4 、 81$ perfect squares). The sum of all four-digit numbers that satisfy this property is $\qquad$