#include <bits/stdc++.h>

inline long nextInt(void) {
  long temp;
  std::cin >> temp;
  return temp;
}

int main() {

  long R, C;
  long ans;

  std::cin >> R >> C;

  if( R == C ) {
    if( R % 2 == 0 ) {
      ans = R * C / 4;
    }
    else {
      ans = (R * C - 1) / 4 + 1;
    }
  }
  else {
    if( (R * C) % 2 == 0 ) {
      ans = R * C / 2;
    }
    else {
      ans = (R * C - 1) / 2 + 1;
    }
  }

  std::cout << ans - 1 << std::endl;
  
  return 0;
}