#include<iostream>
using namespace std;
class Triangle{
    int base,height;
public:
  Triangle(innt b,int h){
      base=b;
      height=h;
  }
  int getArea(){
      return(base*height)/2;
  }
};
int main(){
    int base,height;
    cin>>base>>height;
    if(base<0||height<0){
        cout << "Invalid input";
        return 0;
    }
    Triangle t(base,height);
    cout << t.getArea();
    retuen 0;
}