Let the line \( x+y=1 \) meet the circle \( x^2+y^2=4 \) at the points \( A \) and \( B \). If the line perpendicular to \( AB \) and passing through the mid point of the chord \( AB \) intersects the circle at \( C \) and \( D \), then the area of the quadrilateral \( ADBC \) is equal to:
