#include <bits/stdc++.h>
#pragma comment(linker, "/stack:64000000")
using namespace std;
template <typename first>
inline first abs(const first& a) {
  return a < 0 ? -a : a;
}
template <typename first>
inline first sqr(const first& a) {
  return a * a;
}
const int INF = INT_MAX / 2;
const long double EPS = 1e-9;
const long double PI = 3.1415926535897932384626433832795;
int main() {
  int n, m;
  cin >> n >> m;
  cout << (n * m) / 2 << endl;
  return 0;
}
