//include //------------------------------------------ #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include using namespace std; //typedef //------------------------------------------ typedef vector VI; typedef vector VVI; typedef vector VS; typedef pair PII; typedef long long LL; //container util //------------------------------------------ #define ALL(a) (a).begin(),(a).end() #define RALL(a) (a).rbegin(), (a).rend() #define PB push_back #define MP make_pair #define SZ(a) int((a).size()) #define EACH(i,c) for(typeof((c).begin()) i=(c).begin(); i!=(c).end(); ++i) #define EXIST(s,e) ((s).find(e)!=(s).end()) #define SORT(c) sort((c).begin(),(c).end()) //repetition //------------------------------------------ #define FOR(i,a,b) for(int i=(a);i<(b);++i) #define REP(i,n) FOR(i,0,n) //constant //-------------------------------------------- const double EPS = 1e-10; const double PI = acos(-1.0); int main(){ cin.tie(0); ios_base::sync_with_stdio(false); LL R, C; cin >> R >> C; if(R != C) cout << (R*C%2==0? ((R*C)/2-1): R*C/2)<< endl; else cout << (R*C%2==0? R*C/4-1: R*C/4) << endl; return 0; }