#include <bits/stdc++.h>
template <typename T>
T gcd(T a, T b) {
  return b == 0 ? a : gcd(b, a % b);
}
template <typename T>
T lcm(T a, T b) {
  return a * b / gcd(a, b);
}
template <typename T>
T clamp(T val, T min, T max) {
  return val < min ? min : (max < val ? max : val);
}
template <typename T>
size_t max_index(T begin, T end) {
  return distance(begin, max_element(begin, end));
}
template <typename T>
size_t min_index(T begin, T end) {
  return distance(begin, min_element(begin, end));
}
using namespace std;
signed main(void) {
  cin.tie(0);
  ios::sync_with_stdio(false);
  int num = 0;
  int n;
  string s;
  cin >> s;
  n = s.length();
  auto found = s.find("01");
  while (found != string::npos) {
    s.erase(found, 2);
    num += 2;
    found = s.find("01");
    if (found == string::npos) {
      found = s.find("10");
    }
  }
  cout << num << endl;
  return 0;
}
