#include <bits/stdc++.h>
using namespace std;
const int maxn = 1e5 + 5;
const int inf = 1000000000;
const int MOD = 1e9 + 7;
const double PI = 3.14159265358979323846264338;
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  long long a, b;
  cin >> a >> b;
  cout << (a * b + 1) / 2 << "\n";
}
