#include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #define REP(i, n) for (int (i) = 0; (i) < (n); (i)++) #define FOR(i, a, b) for (int (i) = (a); (i) < (b); (i)++) #define RREP(i, a) for(int (i) = (a)-1; (i) >= 0; (i)--) #define FORR(i, a, b) for(int (i) = (a)-1; (i) >= (b); (i)--) #define PI acos(-1.0) #define DEBUG(C) cerr << C << endl; #define VI vector #define VII vector #define VL vector #define VLL vector #define VD vector #define VDD vector #define PII pair #define PDD pair #define PLL pair #define VPII vector #define ALL(a) (a).begin(), (a).end() #define SORT(a) sort(ALL(a)) #define REVERSE(a) reverse(ALL(a)) #define MP make_pair #define EB emplace_back #define FORE(a, b) for (auto &&a : b) #define FIND(s, n) (s.find(n) != s.end()) using namespace std; typedef long long LL; typedef unsigned long long ULL; const int INF = 1e9; const int MOD = INF + 7; const LL LLINF = 1e18; int main(void) { LL R, C; cin >> R >> C; LL ans = 0; if (R % 2 == 0 && C % 2 == 0) { ans = R * C / 2; } else if (R % 2 == 0 || C % 2 == 0) { ans = R * C / 2; } else { ans = (R * C + 1) / 2; } cout << ans - 1 << endl; }