#include <bits/stdc++.h>
using namespace std;
template <typename T>
inline T abs(T a) {
  return ((a < 0) ? -a : a);
}
template <typename T>
inline T sqr(T a) {
  return a * a;
}
const int INF = (int)1E9 + 7;
const long double EPS = 1E-9;
const long double PI = 3.1415926535897932384626433832795;
int n, m;
pair<int, int> a[100];
int solve(int x, int y, int i) {
  int ans = i;
  if (y == m) x++, y = 0;
  if (x == n) return ans;
  ans = max(ans, solve(x, y + 1, i));
  bool ok = true;
  for (int j = 0; j < int(i); ++j) {
    if (sqr(a[j].first - x) + sqr(a[j].second - y) == 5) {
      ok = false;
      break;
    }
  }
  if (ok) {
    a[i] = make_pair(x, y);
    ans = max(ans, solve(x, y + 1, i + 1));
  }
  return ans;
}
long long solve(long long n, long long m) {
  if (n < m) swap(n, m);
  if (m > 2) return (n * m) / 2 + ((n * m) & 1);
  if (m == 1) return n;
  int k = (n - 2) / 4, rem = (n - 2) % 4;
  int start = 4 + k * 4;
  if (rem < 3) return start;
  return start + 2;
}
int main() {
  int n, m;
  cin >> n >> m;
  cout << solve(n, m) << endl;
  return 0;
}
