Let a, b, c, d be distinct non - zero real numbers with a + b = c + d. Then an eigenvalue of the matrix \( \begin{bmatrix} {a} & {b} & 1 \\[0.3em] c & d & 1 \ \\[0.3em] 1& -1& 0 \\[0.3em] \end{bmatrix} \) is