Question: double integral and volume

Graph the solid bounded by the plane x+y+z =1 and paraboloid z=4-x^2 -y^2, and find the volume. here what i tried, but i cannot get the volume > f := x+y-1; > g := 4-x^2-y^2; > plot3d({f, g}, x = -2 .. 2, y = -sqrt(1-x^2) .. sqrt(1-x^2), axes = boxed); V:=Doubleint(g-f,y=-sqrt(1-x^2)..sqrt(1-x^2),x=-2..2): V=value(V);
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