function Main(input){
	'use strict'
	const S = [...input];
	let c0=0;
	let c1=0;

	S.forEach((v)=>{
		if(v=="0") c0++;
		else c1++;
	});
	console.log(Math.min(c0,c1)*2);

}
Main(require("fs").readFileSync("/dev/stdin", "utf8"));