#include <bits/stdc++.h>
using namespace std;
int main() {
  string s;
  cin >> s;
  int c1 = 0, c2 = 0;
  for (int i = 0; i < s.size(); i++) {
    if (s[i] == '0')
      c1++;
    else
      c2++;
  }
  cout << min(c1, c2) * 2 << endl;
}
