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

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