#include <bits/stdc++.h>
using namespace std;
int main() {
  string s;
  cin >> s;
  int c0 = 0, c1 = 0;
  for (int i = 0; i < s.size(); i++) {
    if (s[i] == '0') c0++;
    c1++;
  }
  cout << 2 * min(c0, c1) << endl;
}
