"use strict"
function main(arg) {
	let a = arg.split(' ');

	let x = a[0];
	let y = a[1];

if(x % 2 === 1)
{
	console.log((parseInt( x / 2) + 1) * y);
}
else{
	console.log((parseInt( x / 2)) * y);

}



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