#include <bits/stdc++.h>
using namespace std;
int main(void) {
  string s;
  cin >> s;
  int a = 0, b = 0;
  for (int i = 0; i < (int)s.size(); ++i) {
    char ch = s[i];
    if (atoi(&ch) == 0) {
      a++;
    } else {
      b++;
    }
  }
  cout << min(a, b) * 2;
}
