#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, m, ans;
  cin >> n >> m;
  if (n < m) swap(n, m);
  if (m == 1) {
    cout << n;
    return 0;
  }
  if (m == 2) {
    ans = 2 * (2 * (n / 4) + min(n % 4, 2));
    cout << ans;
    return 0;
  }
  if (n & 1 && m & 1) {
    ans = ((n - 1) / 2) * m + (m / 2) + 1;
  } else {
    if (!(n & 1)) {
      ans = m * (n / 2);
    } else {
      ans = n * (m / 2);
    }
  }
  cout << ans;
  return 0;
}
