// By rbtree - Koi Carp
// Please submit with C++14!
// :(     :\     :/     :)
#include <algorithm>
#include <array>
#include <cmath>
#include <functional>
#include <iostream>
#include <list>
#include <map>
#include <queue>
#include <set>
#include <tuple>
#include <unordered_map>
#include <utility>
#include <vector>
#define ra _Read_Int()
#define rc _Read_Char()
#define rs _Read_String()
#define iotop _Get_IO_Top()
#ifdef ___RB_DEBUG___
#include "rb_debug.h"
#else
#define dbg(...)
#define dputs(...)
#endif

using tp = long long;
tp _Read_Int();
char _Read_Char();
bool _Get_IO_Top();
std::string _Read_String();
using namespace std;
constexpr bool __MTCS__ = 0;
constexpr tp _BUF_SIZE_ = 2172179;

namespace __SOL__ {
constexpr tp Hat_S = 40;
tp tar = -1ull >> 2, n, m;

array<array<bool, Hat_S>, Hat_S> cd;

void dfs(tp x, tp y, tp cnt) {
  bool a = cd[x - 1][y], b = cd[x][y - 1], c = cd[x][y], d = cd[x + 1][y], e = cd[x][y + 1];
  if (cnt >= tar) {
    return;
  }
  if (x > n) {
    for (tp i = 1; i <= n; ++i) {
      for (tp j = 1; j <= m; ++j) {
        if (!cd[i][j]) {
          return;
        }
      }
    }
    tar = cnt;
    return;
  }
  if (x != 1 && x < n && !cd[x - 1][y]) {
    cd[x - 1][y] = cd[x][y - 1] = cd[x][y] = cd[x + 1][y] = cd[x][y + 1] = 1;
    dfs(x + (y == m), y == m ? 1 : y + 1, cnt + 1);
  } else if (x == n && y != m && y != 1 && (!cd[x - 1][y] || !cd[x][y - 1])) {
    cd[x - 1][y] = cd[x][y - 1] = cd[x][y] = cd[x + 1][y] = cd[x][y + 1] = 1;
    dfs(x, y + 1, cnt + 1);
  } else if (x == n && y == m &&
             (!cd[x - 1][y] || !cd[x][y - 1] || !cd[x][y])) {
    cd[x - 1][y] = cd[x][y - 1] = cd[x][y] = cd[x + 1][y] = cd[x][y + 1] = 1;
    dfs(x + 1, 1, cnt + 1);
  } else {
    dfs(x + (y == m), y == m ? 1 : y + 1, cnt);
    cd[x - 1][y] = cd[x][y - 1] = cd[x][y] = cd[x + 1][y] = cd[x][y + 1] = 1;
    dfs(x + (y == m), y == m ? 1 : y + 1, cnt + 1);
  }
  cd[x - 1][y] = a;
  cd[x][y - 1] = b;
  cd[x][y] = c;
  cd[x + 1][y] = d;
  cd[x][y + 1] = e;
}

signed main([[maybe_unused]] size_t __CASE__) {
  n = ra;
  m = ra;
  if (n > m) {
    swap(n, m);
  }
  if (n == 1) {
    printf("%lld
", m - (m + 2) / 3);
    return 0;
  }
  dfs(1, 1, 0);
  printf("%lld
", n * m - tar);
  return 0;
}
}  // namespace __SOL__

signed main() {
  tp __MTCS__ = ::__MTCS__ ? ra : 1;
  for (tp __i = 1; __i <= __MTCS__; ++__i) {
    __SOL__::main(__i);
  }
  return EXIT_SUCCESS;
}

namespace _READ_RAW_ {
array<char, _BUF_SIZE_> _BUF_;
array<char, _BUF_SIZE_>::iterator __Cur, __End = __Cur + 1;

char _Rd() {
  if (++__Cur == __End) {
    __Cur = _BUF_.begin();
    __End = __Cur + fread(_BUF_.begin(), 1, _BUF_SIZE_, stdin);
  }
  return *__Cur;
}
}  // namespace _READ_RAW_

bool _Get_IO_Top() {
  if (_READ_RAW_::__Cur + 1 == _READ_RAW_::__End) {
    _READ_RAW_::__Cur = _READ_RAW_::_BUF_.begin();
    _READ_RAW_::__End = _READ_RAW_::__Cur +
                        fread(_READ_RAW_::_BUF_.begin(), 1, _BUF_SIZE_, stdin);
    return _READ_RAW_::__Cur-- != _READ_RAW_::__End;
  }
  return 1;
}
tp _Read_Int() {
  bool __neg(0);
  char __c(_READ_RAW_::_Rd());
  tp __val(0);
  for (; __c < 48 || __c > 57; __c = _READ_RAW_::_Rd()) {
    __neg = __c == 45;
  }
  __val = __c & 15;
  for (__c = _READ_RAW_::_Rd(); __c > 47 && __c < 58; __c = _READ_RAW_::_Rd()) {
    __val = __val * 10 + (__c & 15);
  }
  return __neg ? ~__val + 1 : __val;
}
char _Read_Char() {
  char __c = _READ_RAW_::_Rd();
  while (__c == 32 || __c == 10 || __c == 13) {
    __c = _READ_RAW_::_Rd();
  }
  return __c;
}
std::string _Read_String() {
  char __c = _READ_RAW_::_Rd();
  string __val;
  while (__c == 32 || __c == 10 || __c == 13) {
    __c = _READ_RAW_::_Rd();
  }
  do {
    __val.push_back(__c);
    __c = _READ_RAW_::_Rd();
  } while (__c != 32 && __c != 10 && __c != 13);
  return __val;
}

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