#include <bits/stdc++.h>
using namespace std;
int main() {
  string s;
  cin >> s;
  int asum, bsum;
  for (int i = 0; i < s.size(); i++) {
    if (s.at(i) == '1') {
      asum++;
    } else {
      bsum++;
    }
  }
  cout << 2 * min(asum, bsum) << endl;
}
