#include <bits/stdc++.h>
using namespace std;
int main() {
  char name[100000];
  vector<char> s;
  int count0 = 0, count1 = 0;
  for (int i = 0; name[i] != '\0'; ++i) {
    cin >> name[i];
    s.push_back(name[i]);
  }
  for (unsigned int i = 0; i < s.size(); ++i) {
    if (s[i] == '0')
      ++count0;
    else if (s[i] == '1')
      ++count1;
  }
  int ans = 2 * min(count0, count1);
  cout << ans << endl;
  return 0;
}
