#include <bits/stdc++.h>
#pragma comment(linker, "/STACK:200000000")
using namespace std;
template <typename T>
inline T Abs(T x) {
  return (x >= 0) ? x : -x;
}
template <typename T>
inline T sqr(T x) {
  return x * x;
}
template <typename T>
string toStr(T x) {
  stringstream st;
  st << x;
  string s;
  st >> s;
  return s;
}
inline int nextInt() {
  int x;
  if (scanf("%d", &x) != 1) throw;
  return x;
}
inline long long nextInt64() {
  long long x;
  if (scanf("%I64d", &x) != 1) throw;
  return x;
}
inline double nextDouble() {
  double x;
  if (scanf("%lf", &x) != 1) throw;
  return x;
}
const int INF = (int)1E9;
const long long INF64 = (long long)1E18;
const long double EPS = 1E-9;
const long double PI = 3.1415926535897932384626433832795;
bool bit(int mask, int x) { return ((mask & (1 << x)) != 0); }
int bitCount(int mask) {
  int ans = 0;
  while (mask > 0) {
    ans += mask & 1;
    mask >>= 1;
  }
  return ans;
}
int d[50][70][70], n, m, M;
int get(int x, int m1, int m2) {
  if (x == m) return bitCount(m1);
  int &ans = d[x][m1][m2];
  if (ans != -1) return ans;
  ans = INF;
  for (int mask = 0; mask < (int)(1 << n); mask++) {
    int rem = 0, size = 0;
    for (int i = 0; i < (int)(n); i++)
      if (bit(mask, i)) {
        if (i + 1 < n) rem |= 1 << (i + 1);
        if (i > 0) rem |= 1 << (i - 1);
        rem |= 1 << i;
        size++;
      }
    if ((m1 & mask) == m1) {
      int cur = get(x + 1, m2 & (~rem), M & (~mask)) + bitCount(mask);
      ans = min(ans, cur);
    }
  }
  return ans;
}
int main() {
  n = nextInt();
  m = nextInt();
  if (n > m) swap(n, m);
  M = (1 << n) - 1;
  memset(d, 255, sizeof d);
  cout << n * m - get(0, 0, M) << endl;
  return 0;
}
