'use strict';
let execute = input => {
  let sides = input.split(' ').map(el => +el);
  let hypotenuse = Math.max(...sides);
  return (
    sides.filter(el => el != hypotenuse).reduce((sum, el) => sum * el, 1) / 2
  );
};

function Main(input) {
  console.log(execute(input));
}
Main(require('fs').readFileSync('/dev/stdin', 'utf8'));