#include<iostream>
#include<iomanip>
#include<string>
using namespace std;
int main(){
    int base,height;
    if(!(cin >> base >> height)){
        return 1;
    }
    if(base < 0 || height <0 ){
        cout << "Invalid input"<<endl;
    }else{
        int area =(base * height) / 2;
        cout < area << endl;
    }
    return 0;
}