C++ :

#include<bits/stdc++.h>
using namespace std;
double a,b;
int main()
{
	cin>>a>>b;
	printf("%.2f",a*b/2);
	return 0;
}

Pascal :

var d,g,m:real;
begin
  readln(d,g);
  m:=d*g/2;
  writeln(m:0:2);
end.