#include <iostream>
using namespace std;
int main()
{
double n, a, h;
cin >> n;
cin >> a;
cin >> h;
cout<< a*n*h / 2;
return 0;
}
#include <iostream>
using namespace std;
int main()
{
double n, a, h;
cin >> n;
cin >> a;
cin >> h;
cout<< a*n*h / 2;
return 0;
}