#include <bits/stdc++.h>
int main() {
  std::string s;
  std::cin >> s;
  unsigned long count = 0;
  for (char c : s) {
    if (c == '1') {
      count++;
    }
  }
  int t = std::min(count, (s.length() - count));
  t /= 2;
  std::cout << t * 4 << std::endl;
}
