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