#include <bits/stdc++.h>
using namespace std;
template <typename T>
bool InRange(T t, T l, T r) {
  return l <= t && t < r;
}
int main() {
  string S;
  int N = S.length();
  int one = 0;
  for (int i = 0; i < N; i++) one += (S[i] - '0');
  int zero = N - one;
  cout << min(one, zero) * 2 << endl;
  return 0;
}
