#include <bits/stdc++.h>
using namespace std;

int main() {
  string S;
  cin >> S;

  vector<int> cnt(2);
  for (int i = 0; i < S.size(); ++i) {
    cnt[S[i] == '1']++;
  }

  cout << 2 * min(cnt[0], cnt[1]) << endl;
}
