#include <bits/stdc++.h>
using namespace std;

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