#include <bits/stdc++.h>
using namespace std;

int main() {
  string A;
  cin >> A;
  
  int s = 0;
  for (int i = 0; i < A.size(); i++) {
    if (A.at(i) == '0') s++;
  }
  
  cout << min(s, A.size() - s) * 2 << endl;
}