#include <bits/stdc++.h>
using namespace std;
long long power(long long b, long long e, long long MOD) {
  if (b == 0 && e == 0) return 1;
  if (b == 0 && e > 1) return 0;
  if (b > 0 && e == 0) return 1;
  long long r = 1;
  while (e) {
    if (e & 1) {
      r = r * b;
      r %= MOD;
    }
    e >>= 1;
    b = b * b;
    b %= MOD;
  }
  return r;
}
long long gcd(long long a, long long b) {
  if (b == 0)
    return a;
  else
    return gcd(b, a % b);
}
const int INF = INT_MAX;
const int MAX = 100005;
const int MOD = 1e9 + 7;
int main() {
  int m, n;
  scanf("%d", &m);
  scanf("%d", &n);
  cout << n * m / 2 << endl;
  return 0;
}
