#include <cstdio>
#include <cstdint>

int main() {
  intmax_t R, C;
  scanf("%jd %jd", &R, &C);

  if (R == C) {
    if (R == 2) return puts("0"), 0;
    return !printf("%jd\n", R-1);
  } else {
    printf("%jd\n", (R*C+1)/2-1);
  }
}