#include <bits/stdc++.h>
using namespace std;
void fopen() {
  freopen("input.txt", "r", stdin);
  freopen("output.txt", "w", stdout);
}
int main() {
  long long h, w;
  cin >> h >> w;
  long long p = (h * w);
  long long q = p / 2, r = p % 2;
  cout << q + r;
}
