#include <bits/stdc++.h>
using namespace std;
int main() {
  string s;
  cin >> s;
  int i1 = 0, i2 = 0;
  for (int i = 0; i < s.length(); i++)
    if (s[i] == '1')
      i1++;
    else
      i2++;
  cout << 2 * (max(i1, i2));
  return 0;
}
