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