#include <bits/stdc++.h>
using namespace std;
int main(int argc, const char* argv[]) {
  string s;
  cin >> s;
  int z = 0;
  for (int i = 0; i < s.size(); i++) {
    if (s[i] == '0') {
      z++;
    }
  }
  z = min(z, (int)s.size() / 2);
  cout << z * 2 << endl;
  return 0;
}
