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