#include <bits/stdc++.h>
using namespace std;
long long n, m;
long long ans;
signed main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cin >> n >> m;
  if (m == 1 || n == 1) return cout << 0 << endl, 0;
  for (int i = 1; i <= n; ++i) {
    if (i & 1) {
      if (m & 1ll)
        ans += (m >> 1ll) + 1ll;
      else
        ans += (m >> 1ll);
    } else
      ans += (m >> 1ll);
  }
  return cout << ans << endl, 0;
}
