#include <bits/stdc++.h>
using namespace std;
signed main() {
  string a;
  cin >> a;
  long long b, c;
  for (long long i = 0; i < a.size(); ++i) {
    if (a.at(i) == '0')
      c++;
    else
      b++;
  }
  cout << min(c, b) * 2;
}
