#include <bits/stdc++.h>
using namespace std;
int main() {
  int N, M;
  cin >> N >> M;
  int total = 0;
  if (N == 1 && M > 1)
    total += ((M / 2) * N);
  else if (M == 1 && N > 1)
    total += ((N / 2) * M);
  else {
    if (N % 2 == 0)
      total += ((N / 2) * M);
    else if (M % 2 == 0)
      total += ((M / 2) * N);
    else
      total += ((M / 2) * N + (N / 2));
  }
  cout << total;
  return 0;
}
