#include <bits/stdc++.h>
using namespace std;
template <class T>
void checkmin(T &a, T b) {
  if (b < a) a = b;
}
template <class T>
void checkmax(T &a, T b) {
  if (b > a) a = b;
}
template <class T>
T sqr(T x) {
  return x * x;
}
template <class T>
T gcd(T a, T b) {
  if (a < 0) return gcd(-a, b);
  if (b < 0) return gcd(a, -b);
  return (b == 0) ? a : gcd(b, a % b);
}
template <class T>
void out(vector<T> A, int n = -1) {
  if (n == -1) n = ((int)A.size());
  if (n > ((int)A.size())) n = ((int)A.size());
  cout << "{";
  for (int i = 0; i < n; i++) cout << A[i] << " ";
  cout << "}" << endl;
}
template <class T>
void out(T A[], int n) {
  cout << "{";
  for (int i = 0; i < n; i++) cout << A[i] << " ";
  cout << "}" << endl;
}
string tostring(int v) {
  ostringstream sout;
  sout << v;
  return sout.str();
}
string tostring(long long v) {
  ostringstream sout;
  sout << v;
  return sout.str();
}
int low(int i) { return i & -i; }
int countbit(int i) { return (i == 0) ? 0 : (1 + countbit(i - low(i))); }
int toint(string s) {
  istringstream sin(s);
  int t;
  sin >> t;
  return t;
}
long long toint64(string s) {
  istringstream sin(s);
  long long t;
  sin >> t;
  return t;
}
const double pi = acos(-1.);
const double eps = 1e-11;
int main() {
  int a, b, n, m;
  cin >> a >> b;
  n = min(a, b);
  m = max(a, b);
  if (n == 1)
    cout << m << endl;
  else if (n == 2) {
    int k = m % 4;
    int ans = m / 4 * 4;
    if (k == 1)
      ans += 2;
    else if (k != 0)
      ans += 4;
    cout << ans << endl;
  } else {
    cout << n * m / 2 + (n % 2) * (m % 2) << endl;
  }
  return 0;
}
