#include <bits/stdc++.h>
using namespace std;
int n, m;
int main() {
  cin >> n >> m;
  if (n > m) swap(n, m);
  if (n == 2) {
    cout << (4 * (m / 4) + 2 * min(m % 4, 2)) << endl;
  } else if (n == 1 || m == 1) {
    cout << max(n, m) << endl;
  } else if (n % 2 && m % 2) {
    cout << ((n - 1) * m / 2 + (m + 1) / 2) << endl;
  } else {
    cout << n * m / 2 << endl;
  }
}
