#include <bits/stdc++.h>
using namespace std;
int main() {
  string S;
  cin >> S;
  int l = S.length();
  int count0, count1 = 0;
  for (int i = 0; i < l; i++) {
    if (strncmp("0", &S.at(i), 1) == 0) {
      count0++;
    } else {
      count1++;
    }
  }
  cout << 2 * min(count0, count1) << endl;
}
