#include <bits/stdc++.h>
using namespace std;
int main() {
  string S;
  cin >> S;
  int n, t;
  for (int i = 0; i < S.size(); ++i) {
    if (S.at(i) == '1') {
      n++;
    } else {
      t++;
    }
  }
  cout << 2 * min(n, t) << endl;
}
