'use strict';

function main(input) {
  const i = input.split(' ').map((x) => x | 0);
  if (i[0] * i[1] % 2 == 0) {
    console.log(i[0] * i[1] / 2);
  } else {
    console.log(((i[0] * i[1] / 2) | 0) + 1)
  }
}

main(require('fs').readFileSync('/dev/stdin', 'utf8'));
