#include<iostream>
#include<functional>
using namespace std;
int main(){
  string s;
  int a=0,b=0;
  cin>>s;
  for(int i=0;i<s.size();i++){
    if(s[i]=='0')
      a++;
    else
      b++;
  }
  cout<<min(a,b)*2<<endl;
}