#include <iostream>
using namespace std;

int main() {
  // Read N and M
  int n, m;
  cin >> n >> m;

  // Calculate the total number of seats
  int total_seats = n * m;

  // Calculate the number of security guards needed
  int security_guards = total_seats / 4;

  // Round up the result to the nearest integer
  if (total_seats % 4 != 0) {
    security_guards++;
  }

  // Print the result
  cout << security_guards << endl;

  return 0;
}