$S\to L.L \quad\{ S.val = L_1.val + L_2.val/2^{L_2.nb} \}$
$\quad \quad \mid \; L \quad \{ S.val = L.val \}$
$L\to LB \quad \{ L.val = 2 * L_1.val + B.val,$
$\qquad \qquad \quad L.nb = L_1.nb + B.nb \}$
$\quad \quad \mid \; B\quad \{ L.val = B.val$
$\qquad\qquad \quad L.nb = B.nb \}$
$B \to 0 \quad \{ B.val = 0$
$\qquad\qquad B.nb = 1 \}$
$\quad \quad \mid \; 1 \quad \{ B.val = 1$
$\quad \quad \quad \qquad B.nb = 1 \}$
Here, $val =$ decimal value, $nb =$ number of bits.