#include <bits/stdc++.h>
using namespace std;
int main() {
  long long int a, b;
  cin >> a >> b;
  if (a % 2 == 0 && b % 2 == 0) {
    cout << (a * b) / 2;
  }
  if (a % 2 == 0 && b % 2 == 1) {
    cout << (a * (b - 1)) / 2 + a / 2;
  }
  if (a % 2 == 1 && b % 2 == 0) {
    cout << ((a - 1) * b) / 2 + b / 2;
  }
  if (a % 2 == 1 && b % 2 == 1) {
    cout << ((a - 1) * (b - 1)) / 2 + (a + b) / 2;
  }
  return 0;
}
