#include<bits/stdc++.h>
using namespace std;

typedef long long LL;
LL solve(LL R, LL C) {
  if(R==C) {
    if(R%2==0) return R*C / 4;
    return R*C/4+1;
  }
  LL sum = R*C;
  if(sum%2==0) return R*C/2;
  return R*C/2+1;
  
  
}

int main() {
  LL R,C;
  cin >> R >> C;
  cout << solve(R, C) -1 << endl;
  
  return 0;
}