#include <iostream>
using namespace std;
int main(){
  string s;
  cin >> s;
  int a=0, b=0;
  for(int i=0;i<s.length();i++){
    if(s[i]=='0') a++;
    if(s[i]=='1') b++;
  }
  if(a>b) a = b;
  cout << a*2 << endl;
}