#include <algorithm>
#include <climits>
#include <cstdlib>
#include <vector>
#include <cctype>
#include <deque>
#include <cmath>
#include <queue>
#include <map>
#include <set>
#include <ctime>
#include <stack>
#include <cstdio>
#include <string>
#include <iomanip>
#include <cstring>
#include <iostream>
#include <functional>

using namespace std;

typedef long long ll;


#define LLMAX LLONG_MAX
#define IMAX INT_MAX

ll solve(ll r, ll c) {
  if (r != c) {
    if ((r*c)%2) {
      return r*(c/2)+(r+1)/2-1;
    }else {
      return (r*c)/2-1;
    }
  }else {
    if (r%2) {
      return 1+((r+1)/2)*((r+1)/2-1)-1;
    }else {
      return (r/2)*(r/2)-1;
    }
  }
}

int main() {
  ll r, c;
  cin >> r >> c;
  cout << solve(r, c) << endl;
}