Codeforces_50A

//Codeforces Round 50A
#include <iostream>
using namespace std;
int main()
{
    int m, n, p;
    cin >> m >> n;
    if(m<2 && n<2){cout<<"0";}
    else{
    p = m * n;
    cout << p/2;
    }
    return 0;
}