#include <iostream>
using namespace std;
typedef long long ll;

int main(){
    ll R, C;
    cin >> R >> C;
    
    if(R == C){
        if(R % 2 == 1 && C % 2 == 1){
            cout << (R * C - 1) / 4 << endl;
        } else {
            cout << (R * C) / 4 - 1 << endl;
        }
    } else {
        if(R % 2 == 1 && C % 2 == 1){
            cout << (R * C - 1) / 2 << endl;
        } else {
            cout << (R * C) / 2 - 1 << endl;
        }
    }
    
    return 0;
}