function main(input) { 
  const args=input.split(' ')
  const a = args[0]*args[1]
  if(args[0]%2===0) {
    console.log(a/2)
  } else if(args[0]%2===1){
    const b =Math.round(a/2)
    console.log(b)
  } else {
    console.log(1)
  }
}
main(require('fs').readFileSync('/dev/stdin', 'utf8'));