#include <bits/stdc++.h>
using namespace std;
long long gcd(long long a, long long b) {
  if (a % b == 0)
    return b;
  else
    return gcd(b, a % b);
}
signed main() {
  string s;
  long long a, b;
  cin >> s;
  for (long long i = 0; i < s.size(); ++i) {
    if (s[i] == '1')
      a++;
    else
      b++;
  }
  cout << min(a, b) * 2 << endl;
  return 0;
}
