<?php /*atcoder.php*/

$at = new Atcoder();
$at->main();
unset($at);

class Atcoder{
	private function splitIn(){
		$buf = str_split(fgets(STDIN));
		return $this->killNewLine($buf);
	}
	private function killNewLine($buf = null){
		$buf = str_replace("\r\n", "", $buf);
		$buf = str_replace("\n", "", $buf);
		$buf = str_replace("\r", "", $buf);
		return $buf;
	}

	public function main(){
		$s = $this->splitIn();
		$ans = 0;
		$i = 0;

		while(true){
			if(array_key_exists(($i + 1), $s)){
				if($s[$i] != $s[($i + 1)]){
					unset($s[$i]);
					unset($s[($i + 1)]);
					$s = array_values($s);
					$i = 0;
					$ans += 2;
				}else{
					$i++;
				}
			}else{
				break;
			}
		}
		echo $ans . "\n";
	}
}
?>
