#include <bits/stdc++.h>
using namespace std;
long long INF = 1LL << 60;
long long mod = 1e9 + 7;
int main() {
  long long a, b;
  cin >> a >> b;
  if (a % 2 == 1 && b % 2 == 1)
    cout << (a * b + 1) / 2 << endl;
  else
    cout << a * b / 2 << endl;
}
