program Project1;
var a,b,c:integer;p,s:real;f:text;
begin
assign(f,'input.dat');reset(f);
readln(f,a,b,c);close(f);
assign(f,'output.sol');rewrite(f);
p:=(a+b+c)/2;
s:=sqrt(p*(p-a)*(p-b)*(p-c));
write(f,s:0:0);close(f);
end.