#include <cstdio>
#include <cstdlib>
#include <cstddef>
#include <vector>
#include <algorithm>
#include <cmath>
#include <string>
#include <iostream>
#include <iomanip>
#define L64 long long
#define MOD (1000000007LL)

int main(void)
{
  L64 r, c;
  std::cin >> r >> c;
  if(r != c){
    std::cout << (r * c + 1) / 2 - 1 << std::endl;
  } else {
    std::cout << (r * c + 3) / 4 - 1 << std::endl;
  }
}