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