function Main(input) {
  input = input.split(" ");
  var h = input[0];
  var w = input[1];
  var ans = 0;
  if(h%2==0){
  	ans = h*w/2;
  }else{
  	ans = Math.ceil(h*w/2);
  }
  console.log(ans);
}

Main(require("fs").readFileSync("/dev/stdin", "utf8"));