#include <bits/stdc++.h>
using namespace std;
using P = pair<int, int>;
using ll = long long;
using M = map<int, int>;
int main() {
  string s, t = "";
  cin >> s;
  int a, b;
  for (int i = 0; i < (int)(s.size()); i++) {
    if (s[i] == '0')
      a++;
    else
      b++;
  }
  cout << min(a, b) * 2 << endl;
  return 0;
}
