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

int main() {
  string S; cin >> S;
  int cnt1 = 0;
  for (int i = 0; S[i]; ++i) if (S[i] == '1') cnt1++;
  cout << min(cnt1, (int)S.length() - cnt1) * 2 << endl;
  return 0;
}