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