#include <bits/stdc++.h>
using namespace std;
int main() {
  string s;
  int i, count = 0;
  cin >> s;
  s = s + "2";
  for (i = 0; i < s.size(); i++) {
    if (s[i] == '2' || s[i + 1] == '2') break;
    if (s[i] == '1' && s[i + 1] == '0') {
      s.erase(i, 2);
      i -= 2;
      count += 2;
    } else if (s[i] == '0' && s[i + 1] == '1') {
      s.erase(i, 2);
      i -= 2;
      count += 2;
    }
  }
  cout << count << endl;
}
