#include <bits/stdc++.h>
using namespace std;
const int maxn = 1000000 + 10;
const int INF = 1e9 + 7;
const int MOD = 1e9 + 7;
const double eps = 1e-10;
const double pi = acos(-1);
int T, cases, n, m;
int main() {
  ios::sync_with_stdio(false);
  ;
  cin >> n >> m;
  int ans;
  if (n == 1 || m == 1)
    ans = n * m;
  else if (n == 2 || m == 2) {
    if (m == 2) swap(n, m);
    if (m % 4 == 1)
      ans = m / 4 * 4 + 2;
    else
      ans = (m + 3) / 4 * 4;
  } else
    ans = max((n * m) / 2, (n * m) - (n * m / 2));
  cout << ans << endl;
  return 0;
}
