#include <bits/stdc++.h>

using namespace std;

#define REP(i,n) for(int i=0; i<(int)(n); i++)

int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);

  long long r, c;
  cin >> r >> c;

  if (r == c)
    cout << (r * c + 3) / 4 - 1<< endl;
  else
    cout << (r * c + 1) / 2 - 1 << endl;
  
  return 0;
}