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

int main() {
  string S;
  cin >> S;
  int N = S.size();
  int A = 0;
  for (int i = 0; i < N; i++) {
    if (S.at(i) == '1') A++;
  }
  cout << 2 * min(A, N - A) << endl;
}
