#include <bits/stdc++.h>
using namespace std;
int main() {
  string s;
  cin >> s;
  int c0 = 0;
  int c1 = 1;
  for (char c : s) {
    if (c == '0')
      c0++;
    else
      c1++;
  }
  cout << 2 * min(c0, c1) << endl;
  return 0;
}
