#include <bits/stdc++.h>
using namespace std;
int arr[16][16];
int main() {
  int m, n, c = 0;
  cin >> m >> n;
  for (int i = 0; i < m; i++) {
    for (int j = 0; j < n; j++) {
      if (i % 2 == 0 && j % 2 == 0) arr[i][j] = 1;
      if (i % 2 != 0 && j % 2 != 0) arr[i][j] = 1;
    }
  }
  for (int i = 0; i < m; i++)
    for (int j = 0; j < n; j++)
      if (arr[i][j] == 0) c++;
  cout << c << endl;
  return 0;
}
