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