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