#include <bits/stdc++.h>
using namespace std;
int main() {
  int aka = 0;
  int ao = 0;
  string S;
  cin >> S;
  for (int X = 0; X < S.size(); X++) {
    if (S.at(X) = 0) {
      aka++;
    } else {
      ao++;
    }
  }
  int tou = min(ao, aka);
  cout << tou * 2 << endl;
}
