#include <bits/stdc++.h>
using namespace std;
int main(void){
  int i,r=0,b=0;
  string s;
  cin >> s;
  for (i=0;i<s.size();i++){
    if (s.at(i)=='0') r++;
    else b++;
  }
  cout << min(r,b)*2 << endl;
  return 0;
}