/* -*- coding: utf-8 -*-
 *
 * 84.cc: No.84 悪の算盤 - yukicoder
 */

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<iostream>
#include<string>
#include<vector>
#include<map>
#include<set>
#include<stack>
#include<list>
#include<queue>
#include<deque>
#include<algorithm>
#include<numeric>
#include<utility>
#include<complex>
#include<functional>
 
using namespace std;

/* constant */

/* typedef */

typedef long long ll;

/* global variables */

/* subroutines */

/* main */

int main() {
  ll r, c;
  cin >> r >> c;

  ll ans;

  if (r == c) {
    if ((r & 1) == 0) ans = r * c / 4 - 1;
    else ans = r * c / 4;
  }
  else {
    if ((r & 1) == 0 || (c & 1) == 0) ans = r * c / 2 - 1;
    else ans = r * c / 2;
  }

  printf("%lld\n", ans);
  return 0;
}