"use strict";
function main(input) {
  let kari = input[0].trim().split(" ");

  let h = parseInt(kari[0]);
  let w = parseInt(kari[1]);
  console.log(((h*w)%2===0)?parseInt(h*w/2):parseInt(h*w+1)/2);
}
main(require('fs').readFileSync('/dev/stdin', 'utf8').trim().split("\n"));