#include <iostream>
#include <algorithm>
#include <vector>
#include <string>
#include <cmath>
#include <iomanip>
using namespace std;
typedef long long LL;

int main(){
    LL R, C;
    cin >> R >> C;
    LL ans = 0;
    ans += R/2 * C;
    ans += R%2 * (C/2);
    ans += R%2 * (C%2);
    cout << ans-1 << endl;
    return 0;
}