Let \( S(1, 0) \) and \( S'(0, 1) \) be the foci of an ellipse such that \( SP + S'P = 2 \) for any point \( P \) on the ellipse. If \( A(x_1, y_1) \) and \( A'(x_2, y_2) \) are the end points of the major axis of this ellipse, then \( x_1 + x_2 + y_1 + y_2 = \)