//+
Point(1) = {40, 40, -20, 5};
//+
Point(2) = {60, 40, -20, 5};
//+
Point(3) = {60, 60, -20, 5};
//+
Point(4) = {40, 60, -20, 5};
//+
Line(1) = {4, 1};
//+
Line(2) = {1, 2};
//+
Line(3) = {2, 3};
//+
Line(4) = {3, 4};
//+
Curve Loop(1) = {1, 2, 3, 4};
//+
Plane Surface(1) = {1};
//+
Extrude {0, 0, -40} {
  Surface{1}; 
}