function main(input) {
  const args = input.split(' ');
  const a = parseInt(args[0], 10);
  const b = parseInt(args[1], 10);
  const c = parseInt(args[2], 10)

  if(a<c && b<c){
    console.log(a*b/2)
  }else if(a<b){
    console.log(a*c/2)
  }else{
    console.log(b*c/2)
  }
}

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