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

	for(let i=0,len=S.length;i<len;i++){
		if(S[i]=="0") c0++;
		else c1++;
	}
	console.log(Math.min(c0,c1)*2);

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