#include <bits/stdc++.h>
using namespace std;
const double PI = acos(-1.0);
const double eps = 1e-6;
const long long mod = 1e9 + 7;
const int inf = 0x3f3f3f3f;
const int maxn = 1e5 + 10;
const int maxm = 100 + 10;
void input() {
  freopen(".inp", "r", stdin);
  freopen(".out", "w", stdout);
}
void run_case() {
  int m, n;
  cin >> m >> n;
  cout << m * n / 2;
}
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  ;
  run_case();
  return 0;
}
