#include <bits/stdc++.h>
using namespace std;
double const EPS = 1e-6;
long long const N = 1e5 + 50;
long long const mod = 1e9 + 7;
signed main() {
  long long n, m, a, b;
  cin >> n >> m;
  a = min(n, m);
  b = max(n, m);
  if (a % 2 == 0 || b % 2 == 0) {
    cout << a * b / 2;
    return 0;
  } else {
    cout << ((a * b) - 1) / 2;
  }
}
