#include <iostream>
#include <string>
#include <algorithm>
using namespace std;

int main(){
  string S;
  cin >> S;
  int _0 = 0,_1 = 0;
  for(int n = 0;n < S.size();n++){
    if(S[n] == '0')_0++;
    else _1++;
  }
  cout << 2*min(_1,_0) << endl;
  return;
}