#include<bits/stdc++.h>
using namespace std;
int n;
int wcnt,bcnt;
char c;
int main(){
	while(cin>>c){
		if(c=='0')
		   wcnt++;
		else if(c=='1')
		   bcnt++;
	}
	cout<<min(wcnt,bcnt)*2<<endl;
   return 0;
}
