#include <bits/stdc++.h>
using namespace std;

int main() {
  string s;
  cin>>s;
  int n=s.size();
  int r=0,b=0;
  for(int i=0;i<n;i++){
    if(s[i]=='0') r++;
    else b++;
  }
  cout<<min(r,b)*2<<endl;
  }
