#include <bits/stdc++.h>
using namespace std;
using ll = long long;
string S;
int main(void) {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cin >> S;
  int blue, red;
  for (auto s : S) {
    if (s == '0')
      ++blue;
    else
      ++red;
  }
  cout << min(red, blue) * 2 << endl;
  return 0;
}
