with Ada.Text_IO; use Ada.Text_IO;
with Ada.Integer_Text_IO; use Ada.Integer_Text_IO;

procedure Solve is
   H : Integer;
   W : Integer;
   A : Integer;
   B : Integer;
begin
   Get (H);
   Get (W);
   Get (A);
   Get (B);
   Put_Line (Integer'Image ((H-A)*(W-B)));
end Solve;