var S,p,a,b,c:real;
begin
read(a,b,c,p);
S:=sqr(p(p-a)*(p-b)*(p-c));
write(S);
end.
var S,p,a,b,c:real; begin
write(S); end
var S,p,a,b,c:real;
begin
read(a,b,c,p);
S:=sqr(p(p-a)*(p-b)*(p-c));
write(S);
end.
var S,p,a,b,c:real; begin
read(a,b,c,p);
S:=sqr(p(p-a)*(p-b)*(p-c));
write(S); end