#include <bits/stdc++.h>
using namespace std;
int main() {
  int l, b, M, N;
  cin >> M;
  cin >> N;
  l = std::max(M, N);
  b = std::min(M, N);
  if (b >= 2) {
    cout << (b * (l / 2) + (b / 2) * (l % 2));
  } else
    cout << (b * (l / 2));
  return 0;
}
