#include <iostream>
using namespace std;

#define MIN(X,Y) ((X)<(Y)?(X):(Y))

int main(){
  int r,b;
  char c;
  
  r = b = 0;
  while(cin>>c){
    c=='0'?r++:b++;
  }
  cout << MIN(r,b)*2 << endl;
  return 0;
}