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

template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return 1; } return 0; }

#define FOR(i,a,b) for(ll i=(a);i<(b);++i)
#define ALL(v) (v).begin(), (v).end()
#define p(s) cout<<(s)<<endl
#define p2(s, t) cout << (s) << " " << (t) << endl
#define br() p("")
#define pn(s) cout << (#s) << " " << (s) << endl

int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);

    // input
    ll R, C;
    cin >> R >> C;

    // 回転により中央が不変なので特別扱い
    if(R%2==1 && C%2==1){
        ll area = R*C;
        area--; // center
        area /= 2;
        if(R==C) area /= 2;
        area++; // add center
        p(area-1);
    }else{
        ll ans = (R*C)/2;
        if(R==C) ans /= 2;
        p(ans-1);
    }

    return 0;
}