function main(input) {
  var input=input.trim();
  var a=0,b=0;
  for(var i=0;i<input.length;i++){
    if(input[i] == '0') a++;
    else b++;
  }
  console.log(Math.min(a,b)*2);
}

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