#include <bits/stdc++.h>
using namespace std;
const long long MOD = 1e9 + 7;
int main() {
  std::ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  long long n, m;
  cin >> n >> m;
  if (n > m) {
    swap(n, m);
  }
  long long c = 0, ans = 0;
  c = m / 2;
  m = m - c * (2);
  ans += n * c;
  swap(n, m);
  c = m / 2;
  ans += c * n;
  cout << ans << "\n";
  return 0;
}
