#include <bits/stdc++.h>
const int ms = 1050;
int got[ms][ms];
int dx[] = {5, -5, 0, 0, 1, 1, -1, -1, 2, 2, -2, -2};
int dy[] = {0, 0, 5, -5, 2, -2, 2, -2, 1, -1, 1, -1};
int main() {
  int n, m;
  std::cin >> n >> m;
  int ans = (n * m + 1) / 2;
  if (n < m) {
    std::swap(n, m);
  }
  int cans = 0;
  for (int i = 5; i < n + 5; i++) {
    for (int j = 5; j < m + 5; j++) {
      bool can = true;
      for (int k = 4; k < 12; k++) {
        can = can && !got[i + dx[k]][j + dy[k]];
      }
      if (can) {
        cans++;
        got[i][j] = true;
      }
    }
  }
  ans = std::max(ans, cans);
  std::cout << ans << '\n';
}
