program q;
var a,b,c,d, a1, b1, c1 :integer;
var e,p :real;
begin
a:=0; b:=0; c:=0; d:=0;
read (a , b , c ,d);
a1:=a-d;
b1:= b-d;
c1:= c-d;
p:= (a1+b1+c1)/2;
e:= sqrt(p*(a1-p)*(b1-p)*()c1-p));
write (e:5);
end.
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