var a,b,c,p,s: real;
begin
readln(a,b,c);
p:=(a+b+c)/2;
s:=sqrt(p*(p-a)*(p-b)*(p-c));
writeln(s);
end.
var a,b,c,p,s: real;
begin
readln(a,b,c);
p:=(a+b+c)/2;
s:=sqrt(p*(p-a)*(p-b)*(p-c));
writeln(s);
end.