#include <bits/stdc++.h>
using namespace std;
int a, b;
void special() {
  if (a > b) swap(a, b);
  int ans;
  if (a == 1) {
    ans = b;
  } else {
    assert(a == 2);
    ans = 0;
    int blocks = b / 4;
    ans = blocks * 4;
    int extras = (b % 4);
    extras = min(extras, 2);
    ans += extras * 2;
  }
  cout << ans << endl;
}
int main() {
  cin >> a >> b;
  if (a < 3 || b < 3) {
    special();
    return 0;
  }
  int ans;
  int da = a / 2, db = b / 2;
  ans = da * db * 2;
  int odd = 0, on = 0;
  if (a % 2) {
    odd += b;
    ++on;
  }
  if (b % 2) {
    odd += a;
    ++on;
  }
  if (on == 2) --odd;
  ans += odd / 2;
  if (odd % 2) ++ans;
  cout << ans << endl;
  return 0;
}
