C++ :

#include<bits/stdc++.h>
using namespace std;
double a,b;
int main(){
	cin>>a>>b;
    printf("%.1f",a*b/2);
    return 0;
}

Pascal :

var a,b:real;
begin
  readln(a,b);
  writeln(a*b/2:0:1);
end.