#include<bits/stdc++.h>
using namespace std; 
int main()
{
  int a=0,b=0;
  string s;

  cin>>s;

  for(int i=0;i<s.size();i++){
    if(s[i]=='0') a++;
    else b++;
  }

  cout<<min(a,b)*2<<endl;
  return 0;
}
