Let \( A \) be a \( 2 \times 2 \) real matrix and \( 1 \) be the identity matrix of order \( 2 \). If the roots of the equation \( |A - xI| = 0 \) be \( -1 \) and \( 3 \), then the sum of the diagonal elements of the matrix \( A^2 \) is __________.