#include <bits/stdc++.h>
using namespace std;
int n, m;
int main() {
  int tn, tm, res;
  cin >> tn >> tm;
  n = ((tn) < (tm) ? (tn) : (tm));
  m = ((tn) > (tm) ? (tn) : (tm));
  if (n == 1) {
    cout << m << endl;
    return 0;
  }
  if (n == 2) {
    res = (m / 4) * 4;
    if (m % 4 == 1)
      res += 2;
    else if (m % 4 == 2 || m % 4 == 3)
      res += 4;
    cout << res << endl;
    return 0;
  }
  cout << (n * m + 1) / 2 << endl;
  return 0;
}
