#include #include #include #include #include #include #include #include #include #include #include #include #define REP(i, n) for(int i = 0;i < n;i++) #define REPR(i, n) for(int i = n;i >= 0;i--) #define FOR(i, m, n) for(int i = m;i < n;i++) #define FORR(i, m, n) for(int i = m;i >= n;i--) #define SORT(v, n) sort(v, v+n); #define VSORT(v) sort(v.begin(), v.end()); #define llong long long #define pb(a) push_back(a) //#define INF ((LLONG_MAX) / (2)) using namespace std; typedef pair P; typedef pair LP; typedef pair PP; typedef pair LPP; typedef long long int ll; typedef pair LL_IP; typedef pair LL_LLP; #define INF 1e9+7 int main(){ ll r,c; cin >> r >> c; ll ans = 0; if(r % 2 == 0){ ans = (r/2)*c-1; }else{ ans = (r-1)/2*c + c/2+1 -1; } cout << ans << endl; return 0; }