#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i, s, e) for (int i = (int)(s); i < (int)(e); ++i)
#define all(a) (a).begin(),(a).end()

int main() {
    cin.tie(nullptr);
    ios_base::sync_with_stdio(false);
    
    ll R, C;
    cin >> R >> C;
    
    ll ans;
    if (R == C) {
        if (R % 2 == 0) ans = (R / 2) * (R / 2);
        else ans = (R / 2) * (R / 2 + 1) + 1;
    } else {
        if (R % 2 == 0 && C % 2 == 0) ans = R*C / 2;
        else if (R % 2 == 0) ans = R*(C / 2) + R / 2;
        else if (C % 2 == 0) ans = C*(R / 2) + C / 2;
        else ans = R*(C / 2) + R / 2 + 1;
    }
    cout << ans - 1 << '\n';
}