#include <bits/stdc++.h>
using namespace std;
int main() {
  string s;
  cin >> s;
  int red = 0, blue = 0;
  red = count(s.begin(), s.end(), '0');
  blue = count(s.begin(), s.end(), '1');
  cout << min(red, blue) * 2 << endl;
  return 0;
}
