Two equal sides of an isosceles triangle are along \( -x + 2y = 4 \) and \( x + y = 4 \). If \( m \) is the slope of its third side, then the sum, of all possible distinct values of \( m \), is:
\( 6 \)
\( -6 \)
\( -2\sqrt{10} \)
\( 12 \)