#include <cstdio>
#include <cstdint>

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

  if (R == C) {
    if (R % 2 == 0) return !printf("%jd\n", R*C/4-1);
    printf("%jd\n", (R*C-1)/4);
  } else {
    printf("%jd\n", (R*C+1)/2-1);
  }
}