#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using P = pair<int, int>;
using vi = vector<int>;
using vvi = vector<vi>;
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  string s;
  cin >> s;
  int num = 0;
  num = count(s.begin(), s.end(), '0');
  cout << num * 2 << endl;
}
